## Summary

First slice of `P0-1` from the player perf proposal: lays the foundation for a player perf gate so later PRs can plug in fps / scrub / drift / parity scenarios without rebuilding infrastructure. Ships one smoke scenario (`03-load`, cold + warm composition load) to prove the gate end-to-end on real numbers.

## Why

There was no automated way to catch player perf regressions. Every perf concern in the existing proposal — composition load time, sustained FPS, scrub p95, mirror-clock drift, live-vs-seek parity — needs the same plumbing: a same-origin harness, a Puppeteer runner, a baseline file, a gate that emits structured results, and a CI workflow that runs the right scenarios on the right changes. Building that up-front in one reviewable PR lets every subsequent perf PR (`P0-1b`, `P0-1c`, and beyond) be a 100-line scenario file plus a baseline entry instead of re-litigating the framework.

## What changed

### Harness — `packages/player/tests/perf/server.ts`

- `Bun.serve` on a free port, single same-origin host for the player IIFE bundle, hyperframe runtime, GSAP from `node_modules`, and fixture HTML.
- Same-origin matters: cross-origin would force every probe through `postMessage`, hiding bugs and inflating numbers in ways production never sees. Tests should measure the path the studio editor actually takes.
- Routes:
  - `/player.js` → built IIFE bundle (rebuilt on demand).
  - `/vendor/runtime.js`, `/vendor/gsap.min.js` → resolved from `node_modules` so fixtures don't need to ship copies.
  - `/fixtures/*` → fixture HTML.

### Runner — `packages/player/tests/perf/runner.ts`

- `puppeteer-core` thin wrappers (`launchBrowser`, `loadHostPage`).
- Uses the system Chrome detected by `setup-chrome` in CI rather than the bundled puppeteer revision — keeps the action smaller, lets us pin Chrome version policy at the workflow level, and matches what users actually run.

### Gate — `packages/player/tests/perf/perf-gate.ts` + `baseline.json`

- Loads `baseline.json` (initial budgets: cold/warm comp load, fps, scrub p95 isolated/inline, drift max/p95) with a 10% `allowedRegressionRatio`.
- Per-metric direction (`lower-is-better` / `higher-is-better`) so the same evaluator handles latency and throughput.
- Returns a structured `GateReport` consumed by both the CLI (table output) and `metrics.json` (CI artifact).
- Two modes: `measure` (log only — used during the rollout) and `enforce` (fail the build) — flip per-metric once we trust the signal, without touching the harness.

### CLI orchestrator — `packages/player/tests/perf/index.ts`

- Parses `--mode` / `--scenarios` / `--runs` / `--fixture` in both space- and equals-separated form (so `--scenarios fps,scrub` and `--scenarios=fps,scrub` both work — matches what humans type and what GitHub Actions emits).
- Runs scenarios, runs the gate, and **always** writes `results/metrics.json` with schema version, git SHA, metrics, and gate rows — so failed runs are still investigable from the artifact alone.

### Fixture + smoke scenario

- `fixtures/gsap-heavy/index.html`: 200 stagger-animated tiles, no media. Heavy enough to make load time meaningful, light enough to be deterministic.
- `scenarios/03-load.ts`: cold + warm composition load. Measures from navigation start to player `ready` event, reports p95 across runs.

### CI — `.github/workflows/player-perf.yml`

- `paths-filter` on `player` / `core` / `runtime` — perf only runs when something that could move the needle actually changed.
- Sets up bun + node + chrome, runs perf in `measure` mode on a shard matrix (so future scenarios shard naturally), uploads `metrics.json` artifacts, and a summary job aggregates shard results into a single PR comment.

### Wiring

- `packages/player`: `puppeteer-core`, `gsap`, `@types/bun` devDeps; typecheck extended to cover the perf `tsconfig`; new `perf` script.
- Root `package.json`: `player:perf` workspace script so `bun run player:perf` runs the whole suite locally with the same flags CI uses.
- `.gitignore`: `packages/player/tests/perf/results/`.
- Separate `tests/perf/tsconfig.json` so test code doesn't pollute the package `rootDir` while still being typechecked.

## Test plan

- [x] Local: `bun run player:perf` passes — cold p95 ≈ 386 ms, warm p95 ≈ 375 ms, both well under the seeded baselines.
- [x] Typecheck, lint, format pass on the perf workspace.
- [x] Existing player unit tests (71/71) still green.
- [ ] First CI run after merge will be the real signal: confirms `setup-chrome` works on hosted runners, the shard matrix wires up, and `metrics.json` artifacts upload.

## Stack

Step `P0-1a` of the player perf proposal. The next two slices are content-only — they don't touch the harness:

- `P0-1b` (#400): adds `02-fps`, `04-scrub`, `05-drift` scenarios on a 10-video-grid fixture.
- `P0-1c` (#401): adds `06-parity` (live playback vs. synchronously-seeked reference, compared via SSIM).

Wiring this gate up first means each follow-up is a self-contained scenario file + baseline row + workflow shard.

… 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.