Stop idle renderer allocation from status animations

[nedtwigg]

Summary

• replace the command-mode selection ring's infinite marching animation with a four-cycle burst that restarts only on command entry or selection change
• replace the persistent alert bell's infinite animation with a four-cycle burst and a static 45-degree resting pose
• carry a per-Session latch sequence so an independent alert track can replay the bounded bell without same-track notification spam keeping it alive
• preserve that sequence across the pre-registry/resume path while keeping it out of persisted Session state

Evidence

Measured in focused Chrome for Testing 150:

• Marching ants: the infinite animation caused 60 style recalculations/second, added 3.77 MB of reclaimable embedder heap, and used 24.33 seconds of renderer CPU over five minutes. Pausing only that animation held embedder heap flat (-29 KB) and used 0.017 seconds over a three-minute control.
• Alert bell: four infinite ringing bells added 6.89 MB of embedder memory, 1,127 style recalculations, and 3.99 seconds of renderer CPU over three minutes. Pausing only those animations reduced that to 0.13 MB, two recalculations, and 0.025 seconds.
• After the bell fix: two consecutive three-minute windows had zero live animations, one style recalculation each, under 0.40 MB of non-cumulative embedder drift, and at most 0.024 seconds of renderer CPU.

In both cases JS heap, DOM node count, and listener count stayed flat. The growth was renderer-side work tied to a live CSS animation, matching the issue's wall-clock/no-terminal-output signature.

Tests

• CI=true pnpm test
• focused live Storybook/CDP before, paused-control, and after measurements

Fixes #483

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[dormouse-bot]

One non-blocking note: toPersistedAlertState in lib/src/lib/session-types.ts now duplicates toPersistedAlert in vscode-ext/src/session-state.ts, which hand-rolls the same status/todo/notification projection over the host's AlertState. Folding the latter into the new helper (alert ? toPersistedAlertState(alert) : fallback ?? null) would leave one place to edit the next time AlertState grows a never-persist field, and would also cover the one call in that file that skips the projection entirely — browserPersistedPane(pane, pane.alert ?? null) in refreshSavedSessionStateFromPtys, which writes a pre-this-PR watchingEnabled straight back out. Happy to push that as a commit if you want it here rather than as a follow-up.