Make hand tracking a Frametop component

- Programs: ft-camd (the camera broker), ft-hands (the tracker), and
  ft-handreplay and ft-ringplay for recordings, built by hands/build.sh
  into hands/build/ with one Makefile. The first build fetches ncnn at
  frame-hands' pinned tag and builds it with the same options.
- ft-camd gets its privileges from file capabilities (CAP_SYS_PTRACE,
  CAP_PERFMON, CAP_DAC_READ_SEARCH) that hands/run.sh install sets with
  sudo, and drops them once set up. It still works under sudo. It runs
  on the host, linked statically, as frametop-camd.service. ft-hands
  runs in the dev container as frametop-hands.service. Both start and
  stop with SteamVR.
- Files move to /run/user/UID/frametop/ (cam-ring, hands, gestures),
  not $XDG_RUNTIME_DIR, which a terminal in the Frametop desktop has
  its own of. SIGUSR1 recordings go to ~/.local/share/frametop/hands.
- The calibration is read through /run/host in the container.
- Settings: HANDS_SWAP_SIDES and HANDS_CPUS in frametop.conf.
- install.sh offers hand tracking as an optional last step.
- The container gets jsoncpp-devel, glibc-static, and NumPy and OpenCV
  for the Python tools.
- tools/ring.py reads the ring, and models/NOTICE credits the
  Apache-2.0 models.

Checked: ft-handreplay gives identical summaries and byte-identical
depth dumps to frame-hands' fh-replay on both 2026-09-29 recordings.

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

hands/include/fh_gestures.h

@@ -1,7 +1,7 @@
 /*
- * fh_gestures - pinch state frame-hands' tracker publishes for input: look at something
- * and pinch to click it, pinch and move to drag (the Vision Pro model, with the eye
- * tracker doing the looking). $XDG_RUNTIME_DIR/frame-hands/gestures, next to the hands
+ * fh_gestures - pinch state ft-hands publishes for input: look at something and pinch to
+ * click it, pinch and move to drag (the Vision Pro model, with the eye tracker doing the
+ * looking). /run/user/UID/frametop/gestures, next to the hands
  * file, with the same sequence lock (read seq, copy, read seq again; use the copy only if
  * both reads are the same even number) and the same frame: metres in the head frame at
  * capture time, OpenVR's HMD frame (+x right, +y up, -z forward).

hands/include/fh_hands.h

@@ -1,12 +1,12 @@
 /*
- * fh_hands - the tracked-hands file frame-hands' tracker publishes
- * ($XDG_RUNTIME_DIR/frame-hands/hands, directory mode 0700), rewritten in place
+ * fh_hands - the tracked-hands file ft-hands publishes for ft-screens' hand cutouts
+ * (/run/user/UID/frametop/hands, directory mode 0700), rewritten in place
  * under a sequence lock: read seq, copy, read seq again; use the copy only if
  * both reads are the same even number.
  *
  * Positions are metres in the head frame at capture time, which is OpenVR's HMD
  * frame (+x right, +y up, -z forward). Turn them into the room with the HMD pose
- * at capture_ns (CLOCK_MONOTONIC). Writers: trackd (fh-tracker), tracker/publish.py.
+ * at capture_ns (CLOCK_MONOTONIC). Writer: ft-hands (hands/track/io.cpp).
  */
 
 #pragma once