itx client surface polish (post-#2063)

Rewritten 2026-07-17: most of this task's original content shipped — PR #2048 unified the browser surface (one socket, useItx(slug?)/connectItx(slug)) and PR #2063 moved everything into packages/iterate with the shared core factored once (apiWebSocketUrl, one keeper, iterate/client vs iterate/node keeping the ws import out of browser bundles). What remains is a small polish list collected from #2063's reviews:

• connectItx means two things across sibling entries: iterate/client's keeper slug-dial (connectItx(slug): Promise<ProjectStub>) vs iterate/node's one-shot object-dial (connectItx(input) overloads). Signatures make misuse a type error, but it is now public npm surface — rename one (e.g. node's → dialItx) if/when the node entry is touched.
• closeIterateSession(): the keeper has no shutdown; every keeper-based script must process.exit (documented in client.ts's header). Additive: retire the current generation without redial; define semantics for pending awaiters/firstConnect.
• disposeStub(stub) helper to absorb the repeated (x as Partial<Disposable>)[Symbol.dispose]?.() casts (six sites) so the "capnweb stubs are loosely-typed disposables" acknowledgment lives once.
• serverSnapshot is a React-flavored name exported from the framework-free module — cosmetic rename candidate.
• lostConnectionTimeout-style UX signal (the one idea worth stealing from the Liveblocks survey): a fast "connection is struggling" status, distinct from the reconnect machinery, so UIs can show degraded state before the two-strike verifier settles.
• Singleton → instance evolution note (roadmap, not committed): the field pattern is a client instance behind a Provider or a createHooks(client) factory with a lazy default singleton. Revisit only if a real need appears (tests wanting isolation, multi-deployment tabs).

Pointers: packages/iterate/src/itx/itx-session.ts, itx-react.ts, itx-node-client.ts; #2063's fable/thermo review findings.