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/track/calib.h

@@ -1,4 +1,4 @@
-// Tracking-camera calibration from the headset's factory files (see tracker/calib.py
+// Tracking-camera calibration from the headset's factory files (see tools/calib.py
 // for the conventions): Kannala-Brandt fisheye intrinsics, and each camera's pose in the
 // head frame (OpenVR's: +x right, +y up, -z forward), metres.
 #pragma once
@@ -27,8 +27,8 @@ struct Camera {
 // name: slam_left, slam_right, upper_left, upper_right.
 bool load_calibration(std::map<std::string, Camera> &out, std::string &err);
 
-// The Arcturus color cameras (tracker/calib.py load_color has the conventions), for
-// fh-camd --with-color's images: luma at 1/scale size, recorded as color_video<N>. They're
+// The Arcturus color cameras (tools/calib.py load_color has the conventions), for
+// ft-camd --with-color's images: luma at 1/scale size, recorded as color_video<N>. They're
 // keyed by those recorded names: left_node is passthrough_left, right_node
 // passthrough_right. crop_subtract: image x = sensor x - the calibration's cropRegion.x.
 // tools/check_color.py tells which node is which and which crop reading fits.