refactor(ui): adopt React canary ViewTransition for the session switch

[AprilNEA]

Summary

Replaces the imperative View Transition stopgap that #299 shipped (manual startViewTransition, generation guard, DOM name pairing) with React's official <ViewTransition> component, per the decision to allow the canary channel. Four commits:

1. chore(deps) — catalog react/react-dom → 19.3.0-canary-96fcba90-20260728 (web/desktop renderers only; mobile pins its own react — the stale catalog comment claiming otherwise is fixed). The canary export is unprefixed ViewTransition (stable-track; canary is the channel Next.js ships in production). minimumReleaseAgeExclude entries added; drop back to latest once 19.3 goes stable.
2. refactor(workbench,ui,desktop) — the declarative rewrite, shaped by two source-verified facts:
   • Pairing is strictly mount/unmount-based (react-dom's appearing-map × deletions walk — a persisting boundary never pairs). So the row title's boundary unmounts when its row becomes active (a plain span takes over, visually seamless) while the header's boundary remounts keyed by session — giving forward and return flights for free.
   • zustand rides useSyncExternalStore, which never enters a transition lane — startTransition around a store write does nothing. One useDeferredValue(selectedId) in useWorkbenchSessions bridges the render path into the lane (a documented ViewTransition trigger), and puts the row unmount + header mount in the same deferred commit, which the pairing requires.
   • @linkcode/ui gains a typed named re-export via an in-module declare module 'react' augmentation (@types/react tracks stable). The whole stopgap module + its 7 unit tests are deleted; interruption/coalescing is now React's own.
3. fix(assets) — drive-by: the committed pi closure manifest was stale since the ws 7→8 bump (chore(deps): bump ws from 7.5.11 to 8.21.1 #289) nested ws@8.21.0 copies under the pi closure; closure.test.ts was already red on master. Regenerated.
4. refactor(webview) — extends the boundary to the web shell's title (the render point feat(workbench,ui): matched-geometry view transition on session switch #299's follow-up commit 8e5b485 covered with the now-removed data attribute).

Motion pacing moves to ::view-transition-group(*) on --motion-normal; the app-level reduce-motion preference silences snapshot animations via CSS (the root-attached pseudos are out of reach of the .reduce-motion * reset).

Verification

• Drive probe against the built desktop app (isolated daemon + two real pi threads): 4 sidebar switches → 3 document.startViewTransition invocations (the rapid double-fire coalesced by React), zero console errors.
• react-dom canary pairing semantics read from the installed production build, not guessed.
• biome / eslint (serial — the concurrency OOM is CODE-468) / tsc green; full vitest suite 2056 passed, including the previously-red closure test.
• Hand-feel needs the usual manual pass (switch, rapid clicks, reduce-motion on/off).

Closes CODE-469

[linear-code]

CODE-469

[chatgpt-codex-connector]

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