perf(player): p0-1b perf tests for fps, scrub latency, and media sync drift (#400)

## Summary

Second slice of `P0-1` from the player perf proposal: plugs the three steady-state scenarios — sustained playback FPS, scrub latency, and media-sync drift — into the perf gate that landed in #399. Adds the multi-video fixture they all share, wires three new shards into CI, and seeds one new baseline (`droppedFramesMax`).

## Why

#399 stood up the harness and proved it with a single load-time scenario. By itself that's enough to catch regressions in initial composition setup, but it can't catch the things players actually fail at in production:

- **FPS regressions** — a render-loop change that drops the ticker from 60 to 45 fps still loads fast.
- **Scrub latency regressions** — the inline-vs-isolated split (#397) is exactly the kind of code path where a refactor can silently push everyone back to the postMessage round trip.
- **Media drift** — runtime mirror logic (#396 in this stack) and per-frame scheduling tweaks can both cause video to slip out of sync with the composition clock without producing a single console error.

Each of these is a target metric in the proposal with a concrete budget. This PR turns those budgets into gated CI signals and produces continuous data for them on every player/core/runtime change.

## What changed

### Fixture — `packages/player/tests/perf/fixtures/10-video-grid/`

- `index.html`: 10-second composition, 1920×1080, 30 fps, with 10 simultaneously-decoding video tiles in a 5×2 grid plus a subtle GSAP scale "breath" on each tile (so the rAF/RVFC loops have real work to do without GSAP dominating the budget the decoder needs).
- `sample.mp4`: small (~190 KB) clip checked in so the fixture is hermetic — no external CDN dependency, identical bytes on every run.
- Same `data-composition-id="main"` host pattern as `gsap-heavy`, so the existing harness loader works without changes.

### `02-fps.ts` — sustained playback frame rate

- Loads `10-video-grid`, calls `player.play()`, samples `requestAnimationFrame` callbacks inside the iframe for 5 s.
- Crucial sequencing: install the rAF sampler **before** `play()`, wait for `__player.isPlaying() === true`, **then reset the sample buffer** — otherwise the postMessage round-trip ramp-up window drags the average down by 5–10 fps.
- FPS = `(samples − 1) / (lastTs − firstTs in s)`; uses rAF timestamps (the same ones the compositor saw) rather than wall-clock `setTimeout`, so we're measuring real frame production.
- Dropped-frame definition matches Chrome DevTools: gap > 1.5× (1000/60 ms) ≈ 25 ms = "missed at least one vsync."
- Aggregation across runs: `min(fps)` and `max(droppedFrames)` — worst case wins, since the proposal asserts a floor on fps and a ceiling on drops.
- Emits `playback_fps_min` (higher-is-better, baseline `fpsMin = 55`) and `playback_dropped_frames_max` (lower-is-better, baseline `droppedFramesMax = 3`).

### `04-scrub.ts` — scrub latency, inline + isolated

- Loads `10-video-grid`, pauses, then issues 10 seek calls in two batches: first the synchronous **inline** path (`<hyperframes-player>`'s default same-origin `_trySyncSeek`), then the **isolated** path (forced by replacing `_trySyncSeek` with `() => false`, which makes the player fall back to the postMessage `_sendControl("seek")` bridge that cross-origin embeds and pre-#397 builds use).
- Inline runs first so the isolated mode's monkey-patch can't bleed back into the inline samples.
- Detection: a rAF watcher inside the iframe polls `__player.getTime()` until it's within `MATCH_TOLERANCE_S = 0.05 s` of the requested target. Tolerance exists because the postMessage bridge converts seconds → frame number → seconds, and that round-trip can introduce sub-frame quantization drift even for targets on the canonical fps grid.
- Timing: `performance.timeOrigin + performance.now()` in both contexts. `timeOrigin` is consistent across same-process frames, so `t1 − t0` is a true wall-clock latency, not a host-only or iframe-only stopwatch.
- Targets alternate forward/backward (`1.0, 7.0, 2.0, 8.0, 3.0, 9.0, 4.0, 6.0, 5.0, 0.5`) so no two consecutive seeks land near each other — protects the rAF watcher from matching against a stale `getTime()` value before the seek command is processed.
- Aggregation: `percentile(95)` across the pooled per-seek latencies from every run. With 10 seeks × 2 modes × 3 runs we get 30 samples per mode per CI shard, enough for a stable p95.
- Emits `scrub_latency_p95_inline_ms` (lower-is-better, baseline `scrubLatencyP95InlineMs = 33`) and `scrub_latency_p95_isolated_ms` (lower-is-better, baseline `scrubLatencyP95IsolatedMs = 80`).

### `05-drift.ts` — media sync drift

- Loads `10-video-grid`, plays 6 s, instruments **every** `video[data-start]` element with `requestVideoFrameCallback`. Each callback records `(compositionTime, actualMediaTime)` plus a snapshot of the clip transform (`clipStart`, `clipMediaStart`, `clipPlaybackRate`).
- Drift = `|actualMediaTime − ((compTime − clipStart) × clipPlaybackRate + clipMediaStart)|` — the same transform the runtime applies in `packages/core/src/runtime/media.ts`, snapshotted once at sampler install so the per-frame work is just subtract + multiply + abs.
- Sustain window is 6 s (not the proposal's 10 s) because the fixture composition is exactly 10 s long and we want headroom before the end-of-timeline pause/clamp behavior. With 10 videos × ~25 fps × 6 s we still pool ~1500 samples per run — more than enough for a stable p95.
- Same "reset buffer after play confirmed" gotcha as `02-fps.ts`: frames captured during the postMessage round-trip would compare a non-zero `mediaTime` against `getTime() === 0` and inflate drift by hundreds of ms.
- Aggregation: `max()` and `percentile(95)` across the pooled per-frame drifts. The proposal's max-drift ceiling of 500 ms is intentional — the runtime hard-resyncs when `|currentTime − relTime| > 0.5 s`, so a regression past 500 ms means the corrective resync kicked in and the viewer saw a jump.
- Emits `media_drift_max_ms` (lower-is-better, baseline `driftMaxMs = 500`) and `media_drift_p95_ms` (lower-is-better, baseline `driftP95Ms = 100`).

### Wiring

- `packages/player/tests/perf/index.ts`: add `fps`, `scrub`, `drift` to `ScenarioId`, `DEFAULT_RUNS`, the default scenario list (`--scenarios` defaults to all four), and three new dispatch branches.
- `packages/player/tests/perf/perf-gate.ts`: add `droppedFramesMax: number` to `PerfBaseline`. Other baseline keys for these scenarios were already seeded in #399.
- `packages/player/tests/perf/baseline.json`: add `droppedFramesMax: 3`.
- `.github/workflows/player-perf.yml`: three new matrix shards (`fps` / `scrub` / `drift`) at `runs: 3`. Same `paths-filter` and same artifact-upload pattern as the `load` shard, so the summary job aggregates them automatically.

## Methodology highlights

These three patterns recur in all three scenarios and are worth noting because they're load-bearing for the numbers we report:

1. **Reset buffer after play-confirmed.** The `play()` API is async (postMessage), so any samples captured before `__player.isPlaying() === true` belong to ramp-up, not steady-state. Both `02-fps` and `05-drift` clear `__perfRafSamples` / `__perfDriftSamples` *after* the wait. Without this, fps drops 5–10 and drift inflates by hundreds of ms.
2. **Iframe-side timing.** All three scenarios time inside the iframe (`performance.timeOrigin + performance.now()` for scrub, rAF/RVFC timestamps for fps/drift) rather than host-side. The iframe is what the user sees; host-side timing would conflate Puppeteer's IPC overhead with real player latency.
3. **Stop sampling before pause.** Sampler is deactivated *before* `pause()` is issued, so the pause command's postMessage round-trip can't perturb the tail of the measurement window.

## Test plan

- [x] Local: `bun run player:perf` runs all four scenarios end-to-end on the 10-video-grid fixture.
- [x] Each scenario produces metrics matching its declared `baselineKey` so `perf-gate.ts` can find them.
- [x] Typecheck, lint, format pass on the new files.
- [x] Existing player unit tests untouched (no production code changes in this PR).
- [ ] First CI run will confirm the new shards complete inside the workflow timeout and that the summary job picks up their `metrics.json` artifacts.

## Stack

Step `P0-1b` of the player perf proposal. Builds on:

- `P0-1a` (#399): the harness, runner, gate, and CI workflow this PR plugs new scenarios into.

Followed by:

- `P0-1c` (#401): `06-parity` — live playback frame vs. synchronously-seeked reference frame, compared via SSIM, on the existing `gsap-heavy` fixture from #399.

.github/workflows/player-perf.yml

@@ -42,6 +42,15 @@ jobs:
           - shard: load
             scenarios: load
             runs: "5"
+          - shard: fps
+            scenarios: fps
+            runs: "3"
+          - shard: scrub
+            scenarios: scrub
+            runs: "3"
+          - shard: drift
+            scenarios: drift
+            runs: "3"
     steps:
       - uses: actions/checkout@v4