fix(shared): give createPersistentStore a destroy() to dispose its effect.root

The store created an $effect.root for the save-on-change sync but returned no
disposer, so the effect leaked for the life of the process — contradicting
the rule that $effect.root owners must expose destroy(). Capture and expose
the disposer.

- add destroy() to the returned store; covered by tests (flushSync proves the
  save effect runs before destroy and stops after)
- trim the bloated header (two near-duplicate @example blocks) to one concise
  JSDoc — no fluff
- update typographySettings test mocks to satisfy the now-required destroy()

Consumers (LayoutManager, ThemeManager, typographySettings, comparisonStore)
do not yet call it — threading + the createSingleton migration follow.

src/features/AdjustTypography/model/store/typographySettingsStore/typographySettingsStore.test.ts

@@ -51,6 +51,7 @@ describe('TypographySettingsStore - Unit Tests', () => {
     let mockPersistentStore: {
         value: TypographySettings;
         clear: () => void;
+        destroy: () => void;
     };
 
     const createMockPersistentStore = (initialValue: TypographySettings) => {
@@ -70,6 +71,7 @@ describe('TypographySettingsStore - Unit Tests', () => {
                     letterSpacing: DEFAULT_LETTER_SPACING,
                 };
             },
+            destroy() {},
         };
     };
 
@@ -535,6 +537,7 @@ describe('TypographySettingsStore - Unit Tests', () => {
                     mockStorage = v;
                 },
                 clear: clearSpy,
+                destroy() {},
             };
 
             const manager = new TypographySettingsStore(