perf(staged): stop chat pane poll from re-rendering the whole transcript

[matt2e]

Summary

While a session runs, SessionChatPane polls every 500 ms; each tick re-fetched the session's entire ACP metadata history and rebuilt every transcript group with fresh object identities, re-running markdown parse + sanitize, pikchr WASM renders, and raw-payload pretty-printing for the full transcript twice a second. In a WKWebView that synchronous work stalls painting, causing the rhythmic UI freeze (~2x/sec hitches of the thinking spinner and composer caret) in both the note dialog and the session dialog, which embed this shared pane.

Changes

• Incremental metadata fetch: new get_session_acp_metadata_messages_since (store query, Tauri command, web dispatch arm) returns metadata rows past a cursor plus fresh copies of explicitly re-requested ids — rows for tool calls without a terminal status are mutated in place by the message writer, so the frontend re-requests those until they settle.
• Identity-preserving poll: poll() merges incremental metadata updates while preserving row and array identity on quiet ticks, and skips session/queued-message/tail-message reassignment when the data is unchanged.
• Stable transcript groups: stabilizeAcpTranscriptGroups reuses group/item identities across rebuilds so the keyed {#each} only re-evaluates groups that changed.
• Lazy tool-card formatting: collapsed tool cards decide expandability via a cheap toolHasDetails presence check; formatJson/toolResultText/diff formatting now runs only once a card is expanded.
• Caching: renderMarkdown output is cached per content string (LRU), note-indicator scans are cached per message row, and last-group checks go through a value-stable derived so streaming ticks no longer re-run splitAtNoteIndicator for every assistant message.

Testing

• New unit tests in acpTranscript.test.ts covering group stabilization, identity reuse, and toolHasDetails.
• New Rust store tests for the incremental metadata query (cursor filtering, refetch ids, ordering).

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