perf(serve): widen smooth burst to 2900ms — park drifts past the old window

Full-benchy diagnosis (240 layers, 12 selected): the caught parks cluster at the
1900ms window EDGE, and the 228 skipped layers showed NO left excursion anywhere
in 100–1900ms — the head DID park (selected layers prove it), the park had just
drifted past the window on the taller layers. So extend DEFAULT_SMOOTH_BURST_MS
to 2900ms (15 offsets, within the 16 cap) to catch the late parks. To verify on a
fresh print; an adaptive window is the follow-up if the top layers still edge out.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>