Widen post-wheel settle in differential replay

[jserv]

The 3 second budget after "wheel down 5" still flaked the violawww-differential job on heavily loaded GitHub runners. The compat-side initial.png and scrolled.png came back bit-identical (changed_ratio 0.00000), recoverable by a plain rerun. Match the 6 second budget that the smoke replay tests/ui/replays/violawww-scroll.replay already uses for the analogous in-process step.

wait-converge would be the principled replacement but is not yet a drop-in: the xdotool backend silently ignores its arguments and falls back to a 500 ms sleep, and the C-side detector can declare quiet on pre-input idle before vw has produced any Expose/Present activity. Record both gotchas in the comment so the cheap delay bump is not relitigated as a missed wait-converge opportunity.

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Summary by cubic

Increase the post-scroll settle time in the differential replay to 6s to stop flaky failures on busy GitHub runners. This matches the 6s budget used by the in-process smoke replay.

• Bug Fixes

  • tests/ui/replays/violawww-scroll-system.replay: raise delay after "wheel down 5" from 3000 ms to 6000 ms to let events drain and the scroll apply, preventing false identical screenshots.
  • Document why wait-converge isn’t used yet: the xdotool backend ignores args and sleeps 500 ms, and the detector can converge before Expose/Present.

• Refactors

  • scripts/run-ui-replay.py: inline the “focus-at requires a prior wait-window” error message for readability.

^{Written for commit e2ca752. Summary will update on new commits.}