fix(studio): keep dense keyframes readable - #2925
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The row virtualization stack made the timeline publish a viewport snapshot on every scroll frame and swap `renderClipContent` across every mounted clip at gesture start and settle. Both are windowing concessions, and neither was gated on the flag, so the build users actually run paid for them while mounting all 1,000 clips anyway. Measured on a 3,000-clip project: median scroll step 16.6ms to 76.9ms, p95 17.9ms to 189.4ms, 40 long tasks to 247. Gate both on the row virtualization flag. The scroll path now stops at the door when the flag is off, so `isScrolling` stays false and resize-driven and programmatic syncs still publish through the immediate path. The flag moves into its own module: the scroll-viewport hook needs to read it, and the virtualization hook already imports the viewport snapshot type back, which would have closed an import cycle. Also release the perf fixture lease from the fixture rather than from the test-hook effect. Loading a fixture writes player state, which changed that effect's dependency identities and tore it down on the next frame, so the lease was revoked moments after it was taken and live iframe discovery overwrote the fixture before the gate could measure it. The e2e gate gains a flag-off arm (`test:timeline-default`, 1,000 elements) next to the existing flag-on one. It refuses the 50,000-element combination, verifies from the mounted DOM that the server under test matches the requested flag, and skips the DOM-size budgets for the unvirtualized build rather than relaxing them, so a skipped budget never reads as a passed one. Verified against a live Studio dev server on the fixture project: flag off, before: interactionP95 303.1ms, longest task 194ms, 0/5 runs pass flag off, after: interactionP95 33.6ms, longest task 0ms, 5/5 runs pass flag on, after: interactionP95 33.2ms, 4/5 runs pass, exit 0 The flag-on arm's fourth run reproducibly reports a 55-58ms long task against a 50ms budget. That is the residual tail of the window swap itself, tracked separately and not addressed here.
The gate has existed since the row virtualization stack landed but nothing under `.github/` referenced it, so it only ever ran when someone ran it by hand. That is how the flag-off scroll regression reached eight merged-ready PRs without anything noticing. Adds a `studio-timeline-viewport` job that boots two Studio dev servers, one per flag state, and runs both arms of the gate against them. Two servers are needed because row virtualization is read from `import.meta.env` at module load, so one process cannot serve both builds. Scoped to a new `studio` paths filter rather than the broad `code` one: the gate only says anything about `packages/studio`, `packages/core` and `packages/studio-server`. Adds a `ci` tier. It applies the constrained budgets without any emulation, because a hosted runner is already slower and noisier than the machine the strict numbers were recorded on, while the existing `low-resource` tier would throttle it a further 4x and measure the throttle rather than the build. The fixture composition is tracked under `tests/e2e/fixtures` but Studio resolves projects from the gitignored `data/projects`, so the job copies it into place instead of a project directory being committed. Both arms run in about 7 seconds each locally, so the job cost is almost entirely dependency install and the workspace build it shares with `studio-load-smoke`.
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Validation