Hands: pick the cameras by the light, and detect grips

ft-hands tracks with the mono IR cameras in dim light and with every
camera (or the colour pair, HANDS_BRIGHT) in bright light, going by the
colour frames' mean brightness with hysteresis and a 2 s hold
(HANDS_CAMERAS=auto, the default; mono, color and all fix it). Colour
frames are placed on the mono cameras' clock by their dequeue time, and a
view in a camera a step lacks waits for that camera's next frame.

A grip (a closed hand) is a second gesture next to the pinch, in version 2
of the gestures file: every fingertip curled toward the wrist, beginning
only on a hand seen open within a second and held up in front. On the
2026-09-30 lit recording that leaves 6 false grips of 14, all with the
hands on the desk; pinch counts are unchanged. ft-handreplay logs grips
and finger curl, watch_gestures.py shows them, and tools/cut_sets.py
copies a few sets out of a recording.

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

hands/tools/cut_sets.py

@@ -0,0 +1,51 @@
+"""Copy a few frame sets out of a recording (ft-hands --record) into a small one, to look at
+or check elsewhere without moving gigabytes. Plain Python, so it runs on the Frame's host.
+
+usage: python3 tools/cut_sets.py REC_DIR OUT_DIR [--sets N] (8, spread evenly) | [--at I,J,...]
+"""
+import argparse
+import os
+import struct
+
+HDR = struct.Struct('<8sII')
+
+
+def offsets(path):
+    offs, size = [], os.path.getsize(path)
+    with open(path, 'rb') as f:
+        off = 0
+        while off + HDR.size <= size:
+            f.seek(off)
+            magic, _, nbytes = HDR.unpack(f.read(HDR.size))
+            if magic[:7] != b'FHSET01' or off + nbytes > size:
+                break
+            offs.append((off, nbytes))
+            off += nbytes
+    return offs
+
+
+def main():
+    ap = argparse.ArgumentParser()
+    ap.add_argument('rec')
+    ap.add_argument('out')
+    ap.add_argument('--sets', type=int, default=8)
+    ap.add_argument('--at', default='')
+    a = ap.parse_args()
+    src = os.path.join(a.rec, 'sets.bin')
+    offs = offsets(src)
+    if a.at:
+        pick = [int(i) for i in a.at.split(',')]
+    else:
+        n = max(1, min(a.sets, len(offs)))
+        pick = [round(i * (len(offs) - 1) / max(n - 1, 1)) for i in range(n)]
+    os.makedirs(a.out, exist_ok=True)
+    with open(src, 'rb') as f, open(os.path.join(a.out, 'sets.bin'), 'wb') as out:
+        for i in pick:
+            off, nbytes = offs[i]
+            f.seek(off)
+            out.write(f.read(nbytes))
+    print('%d of %d sets (%s) -> %s' % (len(pick), len(offs), ','.join(map(str, pick)), a.out))
+
+
+if __name__ == '__main__':
+    main()