record the full experiment: video, CSV and metadata

Phase 1 of the review asked for one directory per experiment, a force CSV,
video of the pre-processing image, and metadata that ties them together. The
record channel from the previous change already kept the whole session; this
adds the on-disk format.

What
- Add record/experiment.py: ExperimentRecorder writes one directory per
  session containing session.json, forces.csv, frames.csv, events.csv,
  raw_NNNN.mp4 (segmented) and an optional lossless PNG sequence.
- forces.csv uses the reviewed column order
  (session_id, frame_id, t_rel_s, FN_raw, FX_raw, FY_raw, FN_N, FX_N, FY_N,
  valid, calibration_id) and leaves a cell empty when a value is unavailable:
  an uncalibrated newton value or a failed readout is never written as 0.
- frames.csv is the frame-index mapping: frame_id, t_rel_s, acquisition wall
  clock, segment, segment_frame, in_video. forces.csv and frames.csv share the
  packet frame_id and the same t_rel_s origin, so video and data can be
  aligned after dropped or segmented frames.
- session.json carries format version, session id, start/end time in UTC,
  device and config (with a sha256 of the config file), app/shared/SDK
  versions, calibration reference, axis convention, filter parameters, video
  codec and the lossy-MP4 caveat, plus the event log and end-of-run statistics.
  It is written once at start so a crash still leaves identifiable metadata.
- Add a record control to the toolbar: mode (video+data / data only) and a
  start/stop toggle that stays enabled while recording.
- Log zeroing, calibration application and session end as timestamped events.

Why
- The recorder is fed from the worker sink list on the acquisition thread and
  only appends to a bounded queue, so disk I/O can never stall acquisition.
- On_packet copies the raw frame instead of holding a reference: the packet
  frame may alias a buffer the SDK reuses between frames.
- A queue overflow is counted and alarmed (rate limited), and a write or encode
  failure is counted, reported and after 5 consecutive errors stops the
  recording with failed=true rather than silently losing data.

Verification
- shared/tests/test_experiment_recorder.py: 12 cases covering the directory
  layout, the CSV contract, frame-id/time-base agreement, segment rolling,
  data-only and lossless modes, queue overflow accounting, encoder failure,
  metadata fields, crash-before-stop, and that on_packet never blocks.
- tools/ui_check.py gains an "experiment record" section (169 checks, 0
  failures overall), driving a recording through the UI and asserting the
  layout, the empty-cell rule and the event sequence.
- apps/studio/tests/test_ui_smoke.py mirrors those invariants.
- docs/record-channel.md documents the format and the remaining phase-2 work.

Not in this change: CSV export of a selected time range, full-session curve
browsing, calibration diagnostics (outlier exclusion, fit residuals) and
PyOpenGL 3D.

apps/studio/config/locales/en_US.json

@@ -104,6 +104,22 @@
     "fy": "FY",
     "no_data": "—"
   },
+  "record": {
+    "label": "Record",
+    "mode_video_data": "Video + data",
+    "mode_data_only": "Data only",
+    "mode_tip": "Video records the pre-processing camera input (no arrows or text overlays). MP4 is a lossy container, not a lossless copy of the raw data.",
+    "start_label": "● Start recording",
+    "stop_label": "■ Stop recording",
+    "start_tip": "Record this experiment: video, force data and metadata into one directory",
+    "stop_tip": "Stop recording and write session.json (always available)",
+    "requires_run": "Press Run to start acquisition before recording",
+    "started": "Recording started ({mode})",
+    "start_failed": "Could not start recording: {error}",
+    "finished": "Recording finished — {frames} frames / {segments} segment(s) in {path}",
+    "finished_with_problems": "Recording finished with problems: {detail}",
+    "alarm": "Recording problem: {detail}"
+  },
   "cal": {
     "weight": "Weight (N)",
     "duration": "Record duration",