perf(studio): define timeline viewport budgets and fixtures

[miguel-heygen]

What

Define measurable timeline viewport budgets, diagnostics, and deterministic large-project fixtures before changing rendering behavior.

Why

Timeline performance needs an explicit, reproducible contract. Without fixed DOM, latency, memory, and fixture budgets, later virtualization work could appear faster while silently dropping content or moving work elsewhere.

How

• Add one budget module for mounted rows, clips, descendants, interaction latency, frame cadence, long tasks, memory return, and run quorum.
• Add diagnostics that expose mounted timeline work through the existing Studio test hook.
• Add deterministic 1,000- and 50,000-element fixture generation with dense-short coverage.

This is D1 of the independent Family D draft Graphite stack. Its review base is the immutable, PR-less Family C baseline; Families A-C do not need to merge for this PR to be reviewed. It supersedes the owned behavior from legacy PRs #2631 and #2632, which remain open for audit until landing.

Test plan

• Unit tests added/updated
• Manual testing performed
• Documentation updated (not applicable)

Validated on the exact Family D tip with 279 focused tests, the full Studio suite (2,939 passed; 18 todos), Studio Server files tests (67 passed), both package typechecks, oxfmt, oxlint, diff and file-size checks, Fallow with zero introduced findings, and two consecutive strict 50,000-element Chromium passes.

[miguel-heygen]

• perf(studio): virtualize timeline clip windows #2703
• perf(studio): index timeline clip windows #2702
• perf(studio): follow playhead across virtualized rows #2701
• perf(studio): centralize timeline viewport geometry #2700
• refactor(studio): extract timeline render contracts #2699
• refactor(studio): isolate clip drag lifecycle #2698
• test(studio): gate timeline viewport performance in Chromium #2697
• perf(studio): define timeline viewport budgets and fixtures #2696 👈 (View in Graphite)
• codex/review-baseline/studio-timeline-family-c-v2

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