* test(desktop): stress long agent sessions in the multitab perf scenario
--tools seeds every transcript with settled tool rounds and drives the live
stream as a working agent turn (tool calls opened and completed between text
chunks), and each tile reveal is timed to next paint (reveal_max_ms) so deep
transcripts report their mount cost.
* perf(desktop): hold the transcript window cut steady while streaming
A fresh weight-walk per store flush slid the cut forward one message at a
time, ~30x/s, and every slide re-indexed the whole windowed transcript —
each row rendered a different message and the runtime repository took its
O(window) rebuild path instead of the one-message update. advanceTranscriptWindow
anchors the cut to a message id and re-cuts once per ~half page of new
content instead of once per flush.
* perf(desktop): stable rows, stepped backfill, pane-shared render budget
Three thread-list fixes for long streaming sessions: memoize the visible-
groups slice and each turn row so a budget-cut advance no longer re-renders
every mounted turn per streamed token; raise the first-paint backfill in
BACKFILL_STEP slices (one bounded commit per frame) instead of a single
20-to-600 transition whose commit landed as a 780ms freeze mid-stream; and
share RENDER_BUDGET across mounted panes so a 4-way grid mounts a quarter
page per pane instead of 4x the fibers.
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