fix: dispose persistent-store effects; close comparisonStore effect leak

Thread the new createPersistentStore.destroy() through its owners so the
save effect.root is actually torn down: LayoutManager gains destroy();
ThemeManager and typographySettings dispose their #store in their existing
destroy().

comparisonStore had its own leak — a constructor $effect.root whose disposer
was discarded, and a module-level persistent storage created at import. Move
storage into the instance (#storage, created lazily per instance), capture
the effect.root disposer, add destroy(), and adopt createSingleton so reset
runs resetAll() + destroy(). Re-export createSingleton from the $shared/lib
barrel; give the test storage mock a destroy().

src/shared/lib/index.ts

@@ -11,6 +11,7 @@ export {
     createPersistentStore,
     createPerspectiveManager,
     createResponsiveManager,
+    createSingleton,
     createVirtualizer,
     type Entity,
     type EntityStore,
@@ -21,6 +22,7 @@ export {
     type Property,
     type ResponsiveManager,
     responsiveManager,
+    type Singleton,
     type VirtualItem,
     type Virtualizer,
     type VirtualizerOptions,