perf(updater): the update chip pulses forever and keeps an idle app at ~30% of a CPU core #1554

Problem

While the "vX.Y.Z available" chip is shown (UpdateAvailableBadge, title bar), the app animates it forever with _pulse.repeat(reverse: true) (lib/features/updater/update_available_badge.dart). The animation never stops, so the engine produces a frame at the display rate for as long as the window is open, and an otherwise idle app burns about a third of a CPU core.

This is every user who has a newer release available and has not pressed "Remind me later" (the chip stays until the update is installed or dismissed).

Measurements

Profile build of current dev (0.4.22+5), Linux, 120 Hz display, window 2056x2616. CPU time of all threads of the process from /proc/<pid>/task/*/stat, SQL workspace open, nothing happening:

Thread split with the chip: main thread ~1.8-2.0 s of CPU per 8 s (~3.8-4.2 ms per frame), raster ~0.32-0.35 s, GL / compositor threads ~0.1 s.

The same effect showed up when I first measured the real app on 0.4.18, where an update to 0.4.19 existed: 30-38% of a core on the welcome screen with nothing happening, 0.0% after updating.

Wrapping the chip in a RepaintBoundary (tried, reverted) did not help: 29.4% vs 31.4%. The cost is the per-frame pipeline of the whole app (~4 ms main thread per frame), not repainting the chip, so the fix is to produce fewer frames, not to shrink the repaint.

Solution (any of these, ordered by effect)

1. Finite pulse. Pulse a few cycles when the chip appears (e.g. 3 x 1.4 s) and then settle to the static "active" colors. After that the app is idle again (0%).
2. Pause when the window is not visible. No pulse in the background / minimized (AppLifecycleState != resumed, or WidgetsBindingObserver), restart on focus.
3. Optionally re-pulse once per day or on hover, instead of continuously.

Reduced motion already stops it (QueryaMotionLevel.off / disableAnimations); the default Full level should not be a permanent animation either.

Acceptance

• With an update pending and the window idle, CPU drops to ~0% after the initial pulse (measure with the same /proc sampling; add a cpu_idle_update_badge scenario to benchmark/app_perf_bench.dart).
• Existing badge tests (isPulseAnimating, pulseDuration) updated for the finite pulse.
• Chip still catches the eye when it first appears.

Related: #987 (120 Hz epic).