fix(chat): coalesce rapid 'Load older history' clicks and stop re-sort churn

[marcelormendes]

Summary

Fixes the UI flicker caused by a hammered "Load older history" button (loading an old conversation). Three sources of churn, all addressed:

1. Trailing coalesce for loadOlder (ConversationChatPanel.tsx): clicks arriving while an older read is in flight, or during a short settle window right after it resolves, are folded into a single follow-up read. Residual queued clicks can no longer issue one read per click and a new read never fires immediately after a resolve. The coalesce window constant LOAD_OLDER_COALESCE_MS (300ms) is exported for tests.

2. No re-sort churn in the store (conversation-model.ts): replacePage now merges an incoming older/newer page into the existing sequence-sorted item list with a linear two-pointer insert instead of re-sorting the entire collection (O(N log N) localeCompare). Unchanged items keep their exact object identity so memoized turn rendering does not re-run when a page is prepended.

3. Stable scroll observer (ConversationChatPanel.tsx): the follow-latest ResizeObserver is attached once per content element instead of being torn down and recreated on every items/pending identity change; the per-commit layout effect only schedules one requestAnimationFrame scroll per committed batch.

Validation

• npm run typecheck — green.
• npm run lint (biome check .) — green.
• npm test — 587 tests pass (64 files), including new coverage:
  • ConversationChatPanel.test.tsx: trailing-burst coalescing (30 synchronous clicks → one in-flight + one coalesced follow-up read, never 30); existing in-flight dedup test still green.
  • conversation-model.test.ts: prepending older history preserves unchanged item object identity and relative order.
  • ConversationTimeline.performance.test.tsx: historical Markdown is not re-rendered after an older-history prepend.
• npm run site:build — green.
• No lint regressions from biome.

Checklist

• The change is focused and does not include unrelated work.
• Tests cover new behavior or the PR explains why tests are unnecessary.
• npm run verify passes locally.
• No credentials, tokens, private data, or generated build artifacts are included.