fix(react): StrictMode cleanup + relax late-handler guard for re-reg

[ochafik]

Summary

Three React-friendliness fixes following #629/#623:

• useApp cleanup closes the App. Previously the effect cleanup only set mounted = false, so under React StrictMode's dev double-invoke the abandoned first App's PostMessageTransport kept its window message listener and received every host postMessage alongside the second instance (JSONRPC ID collisions, double-fired notifications). Now appInstance?.close() runs in cleanup and after a post-unmount connect resolution. Protocol.close() is idempotent and rejects pending requests with ConnectionClosed, so the abandoned instance's ui/initialize typically isn't even sent. May help with ontoolinput is not consistently called #476 reports from dev environments.
• Late-handler guard only flags the first registration per event. feat(app): warn when one-shot handlers are registered after connect() #629 warned (or threw under strict) on every one-shot handler registration after connect(), which false-positives on the React useEffect([app, dep]) re-registration pattern even when a handler was correctly registered via onAppCreated first. Now a _everHadListener set tracks per-event whether any handler ever existed; re-registration is silent. Only the genuinely-risky "first handler for this event is post-connect" case warns/throws.
• AppBridge warns on a second ui/initialize. Diagnostic only — behavior unchanged (still responds, latest appInfo/appCapabilities win). Surfaces View double-mounting to host implementers.

ONE_SHOT_EVENTS and _everHadListener remain private — not part of the public API.

Test plan

• npm test — 332/332 pass (4 new in late handler registration, 1 close() stops further notification delivery, 1 AppBridge warns on second ui/initialize)
• npx tsc --noEmit — 0 errors
• Re-registration after pre-connect handler: silent, including under strict: true
• First late registration: warns once, subsequent silent; per-event independent
• app.close() stops toolresult delivery
• AppBridge double-init warning fires
• e2e: basic-server-react renders correctly (covers useApp in a real browser)

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commit: 35f4dc8