feat(R3.6): ambient ("Ambilight") + theater/fullscreen modes

[detain]

R3.6 — Ambient ("Ambilight") + theater/fullscreen modes

The player's ambient glow is now live (canvas-sampled from the video frame), and theater mode is wired. Eighth step of the R3 player track; the locked art direction is the R0 mockup src/dev/mockups/player-chrome.html (.player-wrap::before ambient halo), now made dynamic.

Added

• src/components/player/ambient.ts — pure, DOM-free helpers: averageRegion (clamped-bounds RGBA averaging; empty rect → black), sampleAmbient (left/right/center colors), rgbString/rgbaString (alpha-clamped), ambientGradient (the mockup's 3-glow radial layout; intensity scales alpha), isBatterySaving (discharging && level ≤ 0.2), + AMBIENT_SAMPLE_W/H (32×18) and AMBIENT_SAMPLE_INTERVAL_MS (250 ≈ 4 Hz).
• src/components/player/AmbientCanvas.vue — the live "Ambilight" layer. A 32×18 offscreen canvas is sampled on a throttled requestVideoFrameCallback loop (rVFC only fires while the video paints → paused video stops sampling for free; a setInterval fallback while playing when rVFC is absent), painted as a layered radial-gradient glow that spills beyond the framed box. Fully disable-able (off when prefs.atmosphere is false, under reduced-motion, or under a best-effort navigator.getBattery() battery-saver heuristic); degrades to a static fallback glow with no loop under jsdom / SSR / no-canvas / tainted cross-origin frames. pointer-events:none + aria-hidden.

Changed

• Player.vue — restructured the root into a non-clipping positioning wrapper holding <AmbientCanvas> (z 0, behind, negative inset so the halo spills past the frame) and a new .player__stage (z 1, overflow:hidden, the framed black box). Theater mode: the t shortcut + a new control-bar theater button (aria-pressed) toggle a widened, edge-to-edge layout with a brighter ambient surround and emit theater(active: boolean) for the host page to widen its column + dim the surroundings (PlayerPage, R3.9). True fullscreen unchanged; its button stays last.
• index.ts — export AmbientCanvas + the ambient helpers/types.

Acceptance criteria

• ✅ Ambient is canvas-sampled at a throttled cadence (≈4 Hz, 32×18 buffer → well under the <2ms/frame budget) and fully disable-able; static fallback under jsdom/no-canvas.
• ✅ Theater toggles layout (widen + dim) without reload via the t key or the control-bar button and emits its boolean state.
• ✅ Fullscreen still works. Anti-slop clean (icons only).

Out of scope (later R3 steps)

PiP/Media-Session/mini-player (R3.7); resume/up-next + mkv/hevc transcode notice (R3.8); PlayerPage page-level dim/widen + poster backdrop (R3.9).

Gates (run by the coordinator)

• vue-tsc --noEmit clean · vite 8 npm run build clean
• vitest run 1241 passing (+40: ambient 17, AmbientCanvas 19, Player +4)
• Coverage: ambient.ts 100% lines · AmbientCanvas.vue 100% lines (98.07% stmts) · components/player 99.56% lines · overall 97.1% lines
• Anti-slop grep clean over shipped src

Review

Reviewer subagent → 3 findings (all LOW): #1 latent rVFC-handle leak on a mid-loop video swap (FIXED — per-handle rvfcVideo tracking + clean watch restart), #2 two teardown paths not explicitly tested (FIXED — added interval-clear + battery-listener-removal tests), #3 no-change confirmation. Re-verify: NO FINDINGS.

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