Pause frame metrics while previews are hidden

[mattcosta7]

Summary

Make frame timing lifecycle-aware so inactive previews do not create false performance regressions.

• use the first RAF callback only to establish a baseline
• avoid recording a synthetic near-zero startup frame
• pause RAF collection while the preview document is hidden
• resume with a fresh baseline instead of counting the hidden interval as dropped frames
• make repeated collector start/stop calls idempotent

Stack

1. Scope buffered performance metrics to each story #156: story-scoped buffered metrics
2. This PR: frame lifecycle and hidden-preview handling
3. Pause performance work while the panel is hidden #158: visibility-driven collection and live-update lifecycle

Validation

• 54 test files / 762 tests across Chromium, Firefox, and WebKit
• npm run tsc
• npm run lint (one pre-existing unused suppression warning)

[Copilot]

Pull request overview

Makes frame timing lifecycle-aware, preventing hidden previews and startup baselines from producing false regressions.

Changes:

• Establishes fresh RAF baselines on start, reset, and visibility restoration.
• Pauses collection while hidden and makes lifecycle calls idempotent.
• Updates browser tests and release metadata.

Show a summary per file

File	Description
frame-timing-collector.ts	Adds visibility-aware RAF lifecycle handling.
frame-timing-collector.browser.test.ts	Tests baseline and visibility behavior.
collector-manager.browser.test.ts	Adapts frame collection test to baseline seeding.
.changeset/frame-lifecycle.md	Documents the patch release.

Review details

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• Files reviewed: 4/4 changed files
• Comments generated: 0
• Review effort level: Balanced

[Copilot]

Review details

• Files reviewed: 4/4 changed files
• Comments generated: 0 new
• Review effort level: Balanced