feat(CompareBoard): add board store with cycling, persistence, and typography seam

src/features/CompareBoard/model/store/boardStore/boardStore.svelte.test.ts

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+import {
+    afterEach,
+    beforeEach,
+    describe,
+    expect,
+    it,
+} from 'vitest';
+import {
+    __resetBoard,
+    getBoard,
+} from './boardStore.svelte';
+
+beforeEach(() => {
+    localStorage.clear();
+    __resetBoard();
+});
+afterEach(() => __resetBoard());
+
+describe('boardStore', () => {
+    it('starts empty with no focal', () => {
+        const board = getBoard();
+        expect(board.pairings).toEqual([]);
+        expect(board.focalId).toBeNull();
+    });
+
+    it('adds a pairing and makes the first one focal', () => {
+        const board = getBoard();
+        const p = board.addPairing('Inter', 'Lora');
+        expect(board.pairings).toHaveLength(1);
+        expect(board.focalId).toBe(p.id);
+        expect(board.focal).toEqual(p);
+    });
+
+    it('cycles focal forward with wrap', () => {
+        const board = getBoard();
+        const a = board.addPairing('Inter', 'Lora');
+        const b = board.addPairing('Roboto', 'Merriweather');
+        board.setFocal(a.id);
+        board.cycle(1);
+        expect(board.focalId).toBe(b.id);
+        board.cycle(1);
+        expect(board.focalId).toBe(a.id);
+    });
+
+    it('cycles focal backward with wrap', () => {
+        const board = getBoard();
+        const a = board.addPairing('Inter', 'Lora');
+        const b = board.addPairing('Roboto', 'Merriweather');
+        board.setFocal(a.id);
+        board.cycle(-1);
+        expect(board.focalId).toBe(b.id);
+    });
+
+    it('empties the board and clears focal when the last pairing is removed', () => {
+        const board = getBoard();
+        const a = board.addPairing('Inter', 'Lora');
+        board.removePairing(a.id);
+        expect(board.pairings).toEqual([]);
+        expect(board.focalId).toBeNull();
+    });
+
+    it('duplicates a pairing as a distinct card next to the source', () => {
+        const board = getBoard();
+        const a = board.addPairing('Inter', 'Lora');
+        const dup = board.duplicate(a.id);
+        expect(dup.id).not.toBe(a.id);
+        expect(dup.headerFontId).toBe('Inter');
+        expect(board.pairings[1].id).toBe(dup.id);
+    });
+
+    it('swaps one role on the focal pairing', () => {
+        const board = getBoard();
+        const a = board.addPairing('Inter', 'Lora');
+        board.swapFont(a.id, 'body', 'Merriweather');
+        expect(board.focal?.bodyFontId).toBe('Merriweather');
+    });
+
+    it('rewrites the shared specimen (global, not per-pairing)', () => {
+        const board = getBoard();
+        board.addPairing('Inter', 'Lora');
+        board.setSpecimen('header', 'New Header');
+        expect(board.specimen.header).toBe('New Header');
+    });
+
+    it('keeps a focal when the focal pairing is removed', () => {
+        const board = getBoard();
+        const a = board.addPairing('Inter', 'Lora');
+        const b = board.addPairing('Roboto', 'Merriweather');
+        board.setFocal(a.id);
+        board.removePairing(a.id);
+        expect(board.pairings).toHaveLength(1);
+        expect(board.focalId).toBe(b.id);
+    });
+
+    it('exposes default per-role typography', () => {
+        const board = getBoard();
+        expect(board.typo.header.size).toBeGreaterThan(0);
+        expect(board.typo.header.weight).toBeGreaterThan(0);
+        expect(board.typo.body.leading).toBeGreaterThan(0);
+    });
+
+    it('sets one role typography independently via setTypo', () => {
+        const board = getBoard();
+        board.setTypo('header', { size: 64, weight: 700, leading: 1.1, tracking: -0.02 });
+        expect(board.typo.header.size).toBe(64);
+        expect(board.typo.header.weight).toBe(700);
+        // body untouched
+        expect(board.typo.body.size).not.toBe(64);
+    });
+
+    it('persists and rehydrates pairings, focal, and specimen', () => {
+        const board = getBoard();
+        const a = board.addPairing('Inter', 'Lora');
+        board.setSpecimen('body', 'Persisted body');
+        __resetBoard();
+        const restored = getBoard();
+        expect(restored.pairings).toHaveLength(1);
+        expect(restored.pairings[0].id).toBe(a.id);
+        expect(restored.focalId).toBe(a.id);
+        expect(restored.specimen.body).toBe('Persisted body');
+    });
+});

src/features/CompareBoard/model/store/boardStore/boardStore.svelte.ts

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+/**
+ * CompareBoard store — the board singleton.
+ *
+ * Owns the comparison board's business state: the ordered list of Pairings, the
+ * single focal pairing, and the board-global specimen text (header + body).
+ * Persists to localStorage as a compact, URL-encoding-friendly blob.
+ *
+ * Typography is NOT owned here as an AdjustTypography store (features can't
+ * import sibling features). Instead the board holds plain per-role typography
+ * values, fed in via `setTypo` by `widgets/Board` (dependency inversion). Font
+ * resolution/loading (Task 14) and frame measurement (Task 16) layer on later.
+ */
+
+import {
+    DEFAULT_FONT_SIZE,
+    DEFAULT_FONT_WEIGHT,
+    DEFAULT_LETTER_SPACING,
+    DEFAULT_LINE_HEIGHT,
+} from '$entities/Font';
+import {
+    type Pairing,
+    type Role,
+    createPairing,
+    nextFocalId,
+} from '$entities/Pairing';
+import {
+    BOARD_SCHEMA_VERSION,
+    BOARD_STORAGE_KEY,
+    DEFAULT_SPECIMEN,
+} from '$features/CompareBoard/model/const/const';
+import {
+    createPersistentStore,
+    createSingleton,
+} from '$shared/lib';
+import { flushSync } from 'svelte';
+
+/**
+ * Compact persisted board shape. Font ids are abbreviated (`h`/`b`) to keep the
+ * blob small and URL-encoding-friendly; specimen text is in localStorage but is
+ * intentionally excluded from any future URL share.
+ */
+interface PersistedBoard {
+    /**
+     * Schema version (gates migrations / the future URL codec).
+     */
+    v: number;
+    /**
+     * Pairings in board order: surrogate id + the two font ids.
+     */
+    pairings: { id: string; h: string; b: string }[];
+    /**
+     * The focal pairing's id, or null when the board is empty.
+     */
+    focalId: string | null;
+    /**
+     * Board-global specimen text.
+     */
+    specimen: { header: string; body: string };
+}
+
+const emptyBoard = (): PersistedBoard => ({
+    v: BOARD_SCHEMA_VERSION,
+    pairings: [],
+    focalId: null,
+    specimen: { ...DEFAULT_SPECIMEN },
+});
+
+/**
+ * Plain per-role typography values the board renders and measures with. Mirrors
+ * the four axes an `AdjustTypography` store exposes, but as a framework-free
+ * value shape the board owns — the inversion seam (`widgets/Board` pushes the
+ * concrete store's values in via `setTypo`). Not persisted here: the
+ * AdjustTypography stores own typography persistence.
+ */
+export interface RoleTypography {
+    /**
+     * Font size in px (honest, absolute — no responsive multiplier).
+     */
+    size: number;
+    /**
+     * Numeric font weight (100–900).
+     */
+    weight: number;
+    /**
+     * Unitless line-height multiplier.
+     */
+    leading: number;
+    /**
+     * Letter spacing in px.
+     */
+    tracking: number;
+}
+
+const defaultRoleTypography = (): RoleTypography => ({
+    size: DEFAULT_FONT_SIZE,
+    weight: DEFAULT_FONT_WEIGHT,
+    leading: DEFAULT_LINE_HEIGHT,
+    tracking: DEFAULT_LETTER_SPACING,
+});
+
+/**
+ * Singleton board store. Pairings live as a reassigned `$state` array (ordered
+ * cycling needs index order); mutations reassign so Svelte tracks them and
+ * persist synchronously through the persistent store.
+ */
+export class BoardStore {
+    /**
+     * Ordered pairings on the board.
+     */
+    #pairings = $state<Pairing[]>([]);
+    /**
+     * The focal pairing's id, or null when the board is empty.
+     */
+    #focalId = $state<string | null>(null);
+    /**
+     * Board-global specimen text shared by every pairing.
+     */
+    #specimen = $state<{ header: string; body: string }>({ ...DEFAULT_SPECIMEN });
+    /**
+     * Per-role typography, fed in by the widget from the AdjustTypography stores
+     * (dependency-inversion seam). Read by font-loading and frame measurement.
+     */
+    #typo = $state<{ header: RoleTypography; body: RoleTypography }>({
+        header: defaultRoleTypography(),
+        body: defaultRoleTypography(),
+    });
+    /**
+     * localStorage-backed mirror of the board blob.
+     */
+    #storage = createPersistentStore<PersistedBoard>(BOARD_STORAGE_KEY, emptyBoard());
+
+    constructor() {
+        const stored = this.#storage.value;
+        this.#pairings = stored.pairings.map(p => createPairing(p.h, p.b, p.id));
+        this.#focalId = stored.focalId;
+        this.#specimen = { ...stored.specimen };
+    }
+
+    /**
+     * Writes current state back to the persistent store. The persistent store's
+     * own effect flushes to localStorage; `destroy()` forces that flush so
+     * synchronous rehydration (and test teardown) never loses a write.
+     */
+    #persist() {
+        this.#storage.value = {
+            v: BOARD_SCHEMA_VERSION,
+            pairings: this.#pairings.map(p => ({ id: p.id, h: p.headerFontId, b: p.bodyFontId })),
+            focalId: this.#focalId,
+            specimen: { ...this.#specimen },
+        };
+    }
+
+    /**
+     * All pairings in board order (reactive).
+     */
+    get pairings(): readonly Pairing[] {
+        return this.#pairings;
+    }
+
+    /**
+     * The focal pairing's id, or null when empty (reactive).
+     */
+    get focalId(): string | null {
+        return this.#focalId;
+    }
+
+    /**
+     * The focal pairing, or undefined when empty (reactive).
+     */
+    get focal(): Pairing | undefined {
+        return this.#pairings.find(p => p.id === this.#focalId);
+    }
+
+    /**
+     * Board-global specimen text (reactive).
+     */
+    get specimen(): { header: string; body: string } {
+        return this.#specimen;
+    }
+
+    /**
+     * Per-role typography values (reactive). Fed by the widget via `setTypo`.
+     */
+    get typo(): { header: RoleTypography; body: RoleTypography } {
+        return this.#typo;
+    }
+
+    /**
+     * Replaces one role's typography values. Called by `widgets/Board` whenever
+     * the corresponding AdjustTypography store changes (the inversion seam).
+     *
+     * @param role - Which role's typography to set.
+     * @param values - The new typography values for that role.
+     */
+    setTypo(role: Role, values: RoleTypography) {
+        this.#typo = { ...this.#typo, [role]: { ...values } };
+    }
+
+    /**
+     * Adds a pairing to the end of the board. The first pairing added becomes
+     * focal.
+     *
+     * @param headerFontId - Font id for the header role.
+     * @param bodyFontId - Font id for the body role.
+     * @returns The created pairing.
+     */
+    addPairing(headerFontId: string, bodyFontId: string): Pairing {
+        const pairing = createPairing(headerFontId, bodyFontId);
+        this.#pairings = [...this.#pairings, pairing];
+        if (this.#focalId === null) {
+            this.#focalId = pairing.id;
+        }
+        this.#persist();
+        return pairing;
+    }
+
+    /**
+     * Clones a pairing as a distinct card inserted directly after the source, and
+     * makes the clone focal so the user can immediately swap one side.
+     *
+     * @param id - Source pairing id.
+     * @returns The new pairing.
+     */
+    duplicate(id: string): Pairing {
+        const index = this.#pairings.findIndex(p => p.id === id);
+        const source = this.#pairings[index];
+        const dup = createPairing(source.headerFontId, source.bodyFontId);
+        this.#pairings = [
+            ...this.#pairings.slice(0, index + 1),
+            dup,
+            ...this.#pairings.slice(index + 1),
+        ];
+        this.#focalId = dup.id;
+        this.#persist();
+        return dup;
+    }
+
+    /**
+     * Removes a pairing. If the removed pairing was focal, focal moves to a
+     * neighbour so exactly one focal always exists on a non-empty board.
+     *
+     * @param id - Pairing id to remove.
+     */
+    removePairing(id: string) {
+        let nextFocal = this.#focalId;
+        if (this.#focalId === id) {
+            // Pick a neighbour from the still-full ordered list; if the only
+            // candidate is the one being removed, the board becomes empty.
+            const candidate = nextFocalId(this.#pairings.map(p => p.id), id, 1);
+            nextFocal = candidate === id ? null : candidate;
+        }
+        this.#pairings = this.#pairings.filter(p => p.id !== id);
+        this.#focalId = nextFocal;
+        this.#persist();
+    }
+
+    /**
+     * Sets the focal pairing.
+     *
+     * @param id - Pairing id to focus.
+     */
+    setFocal(id: string) {
+        this.#focalId = id;
+        this.#persist();
+    }
+
+    /**
+     * Steps focal one pairing in board order, wrapping at both ends.
+     *
+     * @param direction - +1 for next, -1 for previous.
+     */
+    cycle(direction: 1 | -1) {
+        if (this.#focalId === null) {
+            return;
+        }
+        const next = nextFocalId(this.#pairings.map(p => p.id), this.#focalId, direction);
+        if (next !== null) {
+            this.#focalId = next;
+            this.#persist();
+        }
+    }
+
+    /**
+     * Swaps the font filling one role of a pairing.
+     *
+     * @param id - Pairing id.
+     * @param role - Which role to swap.
+     * @param fontId - New font id for that role.
+     */
+    swapFont(id: string, role: Role, fontId: string) {
+        this.#pairings = this.#pairings.map(p => {
+            if (p.id !== id) {
+                return p;
+            }
+            return role === 'header' ? { ...p, headerFontId: fontId } : { ...p, bodyFontId: fontId };
+        });
+        this.#persist();
+    }
+
+    /**
+     * Rewrites the board-global specimen for a role.
+     *
+     * @param role - Which role's text to set.
+     * @param text - New specimen text.
+     */
+    setSpecimen(role: Role, text: string) {
+        this.#specimen = { ...this.#specimen, [role]: text };
+        this.#persist();
+    }
+
+    /**
+     * Flushes the pending persist write, then disposes the persistent store.
+     * Call on teardown.
+     */
+    destroy() {
+        flushSync();
+        this.#storage.destroy();
+    }
+}
+
+const board = createSingleton(
+    () => new BoardStore(),
+    instance => instance.destroy(),
+);
+
+export const getBoard = board.get;
+
+// test-only reset, so specs don't share live state or persisted blobs
+export const __resetBoard = board.reset;