Switching to a STREAMING session took ~1.4s to settle while an idle
session settled in ~50ms. The autopsy probe named it: the
FIRST_PAINT_BUDGET -> RENDER_BUDGET backfill runs as a transition, an
interrupted transition restarts from scratch, and stream flushes land
every 33-250ms — so the 300-part backfill re-rendered over and over
(measured: 1374ms settle, 30 commits, Primitive.div x2237 for one switch).
Gate the backfill on the thread being idle. The user lands on the live
tail immediately either way; older turns backfill the moment the run
ends, and 'Show earlier' remains the manual path meanwhile.
Measured on the live app (diag-switch-autopsy, real sessions):
switch to idle session ~35-55ms settled (unchanged)
switch to streaming session 1374ms -> backfill deferred; lands at
the live tail like any other switch
Adds diag-switch-autopsy.mjs (per-switch settle/commits/top-renders) and
live-drive.mjs (status/fps/drag one-liners against the running app).
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