lane C: session start — verified frozen contracts against live synthetic API

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

plan/status/lane-C.md

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+# Status — lane-C
+
+## Round 1 — 2026-07-16 — STATUS: in_progress
+
+**Directives acknowledged:** round 0 of plan/DIRECTIVES.md (run order C→A→D→B; C starts
+first as the longest lane + earliest API-contract validation; merge when acceptance passes,
+don't wait on siblings; maintain this status file; evidence discipline — a checked box names
+the test/command+output that proves it).
+
+**Acceptance checklist** (lane brief M4/M5/M6 + timeline markers):
+- [ ] M4 — synchronized playback: master clock; per-video correction (hard-seek >150ms,
+      nudge playbackRate 20–150ms, lock <20ms); out-of-range videos pause+dim; one audio
+      source; dev overlay per-video sync error stays <50ms during continuous playback.
+- [ ] M5 — 3D viewer: PLY point cloud; per-video camera-path lines + current-pose frusta
+      (colored); OrbitControls; snap-to-camera (intrinsics→PerspectiveCamera) matches the
+      video; free-roam detaches. All COLMAP→Three via frozen `lib/pose.js`.
+- [ ] M6 — anchor overlays: per-video transparent canvas, letterbox-correct; project each
+      anchor via pose.js camera; skip if behind (z_cam<0); corners track the stage box in
+      all 3 videos while playing.
+- [ ] Timeline event markers: `GET /api/events` as colored markers (color by event_type,
+      legend); hover tooltip; click-to-jump.
+- [ ] App runs against synthetic data with **no console errors**; merged to `main`.
+
+**Done this round (session start):** verified environment and the frozen contracts against the
+live server (`python -m festival4d serve`): manifest (3 videos, offsets 0/+1370/−842 ms,
+t_global_max 21.37), 41 registered poses/video keyed by frame_idx (centered intrinsics
+fx=fy=554.26, cx=320, cy=180), 4 stage-corner anchors, 7 typed events, Range→206, colored
+PLY (3106 pts). Independently verified stage-corner projection lands all 4 corners in-frame,
+in front of the camera, for all 3 videos (cam0 left, cam1 front-center, cam2 right; centers on
+an arc at radius ~9.5) — so M5/M6 geometry is sound before writing render code.
+
+**Blockers / questions for coordinator:** none. No change requests — the frozen API/pose/
+timebase contracts are sufficient for the whole lane as-is.
+
+**Next:** build in dependency order — state → transport (verify M4 + dev sync overlay first,
+per brief) → videoGrid → timeline → scene3d → overlays; verify live in-browser; adversarial
+review pass; merge.