refactor(hooks): modernize useSyncExternalStore usage and fix useLocalStore tearing

[ctrlplusb]

Summary

Drop the use-sync-external-store polyfill shim and fix a concurrent mode tearing bug in useLocalStore.

Parent issue: #1004

Background

The use-sync-external-store/shim/with-selector polyfill was added for React 16/17 backward compatibility, but peerDependencies already requires ^18.0 || ^19.0. The shim is dead weight. Additionally, useLocalStore still uses the pre-React 18 useState + subscribe pattern which can cause tearing in concurrent mode.

Changes

1. Drop the shim, use native API

• Remove use-sync-external-store dependency
• Import useSyncExternalStore natively from react
• Either use use-sync-external-store/with-selector (non-shim path) or implement selector support manually

2. Remove initializeUseStoreState indirection

• src/hooks.js:4-9 — the dynamic injection pattern was needed for React 16/17; no longer necessary
• Import directly in hooks.js

3. Fix useLocalStore concurrent mode tearing

• src/use-local-store.js:29-36 currently uses useState + useEffect + store.subscribe — the pre-React 18 pattern
• Migrate to useSyncExternalStore for consistency with useStoreState

Testing

• All existing tests should continue to pass
• Add a concurrent mode test for useLocalStore verifying no tearing during rapid updates
• Verify bundle size reduction from dropping the shim