useChat external store on ChatClient (approach 2/3: subscribe/getSnapshot)

[tombeckenham]

Draft / experiment — approach 2 of 3. See #5 for the full writeup and motivation. Base is feat/chat-client-persistence so the diff is only the relevant change.

Promote the external store onto ChatClient

Approach 1 (#5) kept the store inside the React hook. But the Solid hook re-implements the identical seven-channel wiring with createSignal, and Vue/Svelte do the same. This PR moves it onto ChatClient so every framework binding shares one primitive.

ai-client

• Export ChatClientSnapshot (immutable projection of the reactive state) and ChatClientStore { subscribe, getSnapshot }.
• Add a readonly store field to ChatClient, backed by the existing setX mutators — each of setIsLoading / setStatus / setIsSubscribed / setConnectionStatus / setSessionGenerating / setError and the messages handler also calls emitStoreChange(), which rebuilds the cached snapshot and notifies listeners.
• Named store (not subscribe) to avoid colliding with the existing live-subscription subscribe().
• The onXChange callbacks stay intact for existing (Solid/Vue/Svelte/persistor) consumers — the store is purely additive.

ai-react

The hook collapses to:

const snapshot = useSyncExternalStore(
  client.store.subscribe, client.store.getSnapshot, client.store.getSnapshot,
)

No onXChange callbacks wired into React at all — only genuine side-effect callbacks remain.

Trade-off vs approach 3 (@tanstack/store)

Zero new dependencies, but the subscribe/getSnapshot plumbing and snapshot caching are hand-written, and there's no built-in selector support (any change re-renders the whole consumer).

Verification

tsc, ESLint (0 errors), all 376 ai-client + 128 ai-react tests pass. (Typecheck requires building ai-client first, since ai-react resolves it via dist types — CI's test:pr does this via the build dependency.)

🤖 Generated with Claude Code

[github-actions]

🚀 Changeset Version Preview

6 package(s) bumped directly, 5 bumped as dependents.

🟥 Major bumps

Package	Version	Reason
@tanstack/ai-elevenlabs	0.2.17 → 1.0.0	Dependent
@tanstack/ai-openai	0.12.0 → 1.0.0	Dependent
@tanstack/ai-react-ui	0.8.5 → 1.0.0	Dependent
@tanstack/ai-solid-ui	0.7.5 → 1.0.0	Dependent

🟨 Minor bumps

Package	Version	Reason
@tanstack/ai-client	0.15.2 → 0.16.0	Changeset
@tanstack/ai-preact	0.8.2 → 0.9.0	Changeset
@tanstack/ai-react	0.14.2 → 0.15.0	Changeset
@tanstack/ai-solid	0.12.2 → 0.13.0	Changeset
@tanstack/ai-svelte	0.12.2 → 0.13.0	Changeset
@tanstack/ai-vue	0.12.2 → 0.13.0	Changeset

🟩 Patch bumps

Package	Version	Reason
@tanstack/ai-vue-ui	0.2.11 → 0.2.12	Dependent