fix(comparisonStore): preserve stored selection on cold load

The seed-defaults effect fired whenever fontA/fontB were still
undefined, including the window between constructor reading storage
and the per-id batch resolving. On a slow batch or fast catalog the
effect clobbered storage with catalog[0]/catalog[N-1], losing the
user's pick on reload.

Now bails when storage already holds IDs, and reads storage via
untrack so per-font selection writes don't re-trigger the effect.

Adds a deterministic regression test that controls catalog/batch
ordering via mockImplementation timing.

src/widgets/ComparisonView/model/stores/comparisonStore.svelte.ts

@@ -102,7 +102,7 @@ export class ComparisonStore {
         this.#fontsByIdsStore = new FontsByIdsStore(fontAId && fontBId ? [fontAId, fontBId] : []);
 
         $effect.root(() => {
-            // Effect 1: Sync batch results → fontA / fontB
+            // Sync batch results → fontA / fontB
             $effect(() => {
                 const fonts = this.#fontsByIdsStore.fonts;
                 if (fonts.length === 0) {
@@ -124,7 +124,7 @@ export class ComparisonStore {
                 }
             });
 
-            // Effect 2: Trigger font loading whenever selection or weight changes
+            // Trigger font loading whenever selection or weight changes
             $effect(() => {
                 const fa = this.#fontA;
                 const fb = this.#fontB;
@@ -154,24 +154,38 @@ export class ComparisonStore {
                 }
             });
 
-            // Effect 3: Set default fonts when storage is empty
+            // Set default fonts when storage is empty
             $effect(() => {
                 if (this.#fontA && this.#fontB) {
                     return;
                 }
 
-                const fonts = fontCatalogStore.fonts;
-                if (fonts.length >= 2) {
-                    untrack(() => {
-                        const id1 = fonts[0].id;
-                        const id2 = fonts[fonts.length - 1].id;
-                        storage.value = { fontAId: id1, fontBId: id2 };
-                        this.#fontsByIdsStore.setIds([id1, id2]);
-                    });
+                // Don't clobber a pending rehydration - only seed when storage is empty.
+                // Untracked: only the catalog load should drive this effect, not the
+                // user's storage writes that happen as a result of normal selection.
+                const hasStoredSelection = untrack(() => {
+                    return storage.value.fontAId !== null || storage.value.fontBId !== null;
+                });
+
+                if (hasStoredSelection) {
+                    return;
                 }
+
+                const fonts = fontCatalogStore.fonts;
+
+                if (fonts.length < 2) {
+                    return;
+                }
+
+                untrack(() => {
+                    const id1 = fonts[0].id;
+                    const id2 = fonts[fonts.length - 1].id;
+                    storage.value = { fontAId: id1, fontBId: id2 };
+                    this.#fontsByIdsStore.setIds([id1, id2]);
+                });
             });
 
-            // Effect 4: Pin fontA/fontB so eviction never removes on-screen fonts
+            // Pin fontA/fontB so eviction never removes on-screen fonts
             $effect(() => {
                 const fa = this.#fontA;
                 const fb = this.#fontB;