feat(CompareBoard): add fitColumns honest column gating

src/features/CompareBoard/lib/measure/fitColumns.test.ts

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+import {
+    describe,
+    expect,
+    it,
+} from 'vitest';
+import { fitColumns } from './fitColumns';
+
+describe('fitColumns', () => {
+    it('packs as many honest columns as fit, gap-aware', () => {
+        // each needs 600, gap 40, available 1280 -> 1 col=600, 2 cols=1240, 3=1880
+        expect(fitColumns({ naturalWidth: 600, available: 1280, gap: 40, maxColumns: 3 })).toBe(2);
+    });
+    it('never exceeds maxColumns even with room', () => {
+        expect(fitColumns({ naturalWidth: 100, available: 5000, gap: 20, maxColumns: 3 })).toBe(3);
+    });
+    it('never returns less than 1', () => {
+        expect(fitColumns({ naturalWidth: 9000, available: 300, gap: 20, maxColumns: 3 })).toBe(1);
+    });
+    it('fits a column at the exact boundary (inclusive)', () => {
+        // 2 cols: 2*600 + 1*40 = 1240 == available -> fits
+        expect(fitColumns({ naturalWidth: 600, available: 1240, gap: 40, maxColumns: 3 })).toBe(2);
+        // one px short -> only 1
+        expect(fitColumns({ naturalWidth: 600, available: 1239, gap: 40, maxColumns: 3 })).toBe(1);
+    });
+    it('respects a maxColumns of 1 even with unlimited room', () => {
+        expect(fitColumns({ naturalWidth: 100, available: 5000, gap: 20, maxColumns: 1 })).toBe(1);
+    });
+});

src/features/CompareBoard/lib/measure/fitColumns.ts

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+/**
+ * Inputs for column gating.
+ */
+export interface FitColumnsInput {
+    /**
+     * The widest pairing's Pretext natural (shrink-wrap) width in px.
+     */
+    naturalWidth: number;
+    /**
+     * Total available width in px for the columns row.
+     */
+    available: number;
+    /**
+     * Gap in px between columns.
+     */
+    gap: number;
+    /**
+     * Hard cap on columns that still preserve an honest measure (2–3).
+     */
+    maxColumns: number;
+}
+
+/**
+ * How many equal honest columns fit. Uses the real per-pairing required width
+ * (Pretext shrink-wrap) — the 45–75ch rule is only a fallback bound elsewhere.
+ * `n` columns occupy `n*naturalWidth + (n-1)*gap`. Clamped to [1, maxColumns].
+ *
+ * @param input - Natural width, available width, gap, and column cap.
+ * @returns The number of columns that fit, in [1, maxColumns].
+ */
+export function fitColumns({ naturalWidth, available, gap, maxColumns }: FitColumnsInput): number {
+    let fit = 1;
+    for (let n = 2; n <= maxColumns; n++) {
+        if (n * naturalWidth + (n - 1) * gap <= available) {
+            fit = n;
+        } else {
+            break;
+        }
+    }
+    return fit;
+}

src/features/CompareBoard/lib/measure/index.ts

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+export {
+    fitColumns,
+    type FitColumnsInput,
+} from './fitColumns';
+export {
+    measureRoleHeight,
+    type RoleHeightInput,
+} from './measureFrameHeight';