Re-derive of PR #38470 on today's main (its target file desktop-controller.tsx no longer exists after the contrib/ refactor; the three surviving ideas are applied at their new homes):
- incremental-external-store-runtime: the dep-less setAdapter effect ran every render; gate on [runtime, store] — behavior-preserving because __internal_setAdapter early-exits on identical store.
- ChatView is now memo()d, and session-tile hoists its inline arrow props to useCallbacks/module constants so the memo actually holds.
- Render-count regression test (mocked Thread) proves an unrelated parent re-render no longer re-renders the chat shell.
Credit: idea and original implementation by @hdd69 in #38470.
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