Add colour cameras, pinch gestures, and depth measures

- fh-camd --with-color publishes the Arcturus colour pair (luma, half
  size, 30 fps) to the ring, flagged FH_CAM_COLOR. fh-tracker records
  them, and fh-replay --cams mono|color|all tracks with them, using the
  module's EEPROM calibration (load_color_calibration).
  tools/check_color.py checks which node is left and how the crop maps.
- Pinch detection per hand (trackd/pinch.h), published to
  $XDG_RUNTIME_DIR/frame-hands/gestures (include/fh_gestures.h). It has
  begin and end counters, times, the pinch point, and the begin point
  for drags. tools/watch_gestures.py shows it live, and fh-replay
  reports it.
- fh-replay --depth and tools/depth_report.py measure the depth without
  ground truth: noise along the line of sight against across it, the
  one-camera guess, and a simulated camera loss.
- fh-tracker --swap-sides, --ring, --keep-presence, --record-only.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

camd/fhring.h

@@ -34,6 +34,10 @@ enum {
 enum {
     FH_CAM_DARK = 1u << 0,          /* the near-black exposures between this node's */
                                     /* normal frames (fh-camd --with-dark)          */
+    FH_CAM_COLOR = 1u << 1,         /* an Arcturus color camera's luma, downscaled  */
+                                    /* (fh-camd --with-color). Not synced with the  */
+                                    /* mono cameras, and capture_ns is on its own   */
+                                    /* clock: line it up with them by dqbuf_ns      */
 };
 
 typedef struct {