Hands: check the tracking cameras before recording, and watch for losing them

After the headset wakes, XRService sometimes fails to load the colour module's
VCINT FPGA image; then only the two side cameras run, without the IR light,
and the tracker finds no hands. hands/camcheck.py reads XRService's log, the
video nodes it holds and ft-camd's ring, and says ok, degraded or unknown.

The recorder won't start while degraded (--ignore-cameras overrides it), offers
a confirmed SteamVR restart, and stops the first hand-size step when the
tracker sees no hand at all. ft-camwatch (unit file only, not enabled) follows
the log, notifies, and with CAMWATCH_AUTO_RESTART=1 restarts SteamVR when the
headset isn't worn and nothing else uses VR.

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

hands/frametop-camwatch.service

@@ -0,0 +1,21 @@
+# Template: the installer replaces @REPO@ with the repo path on the Frame. Not installed or
+# enabled by anything yet (hands/README.md, "Camera check").
+[Unit]
+Description=Frametop camera watch: tells you when SteamVR leaves the headset's upper cameras off
+Documentation=file://@REPO@/hands/README.md
+# Not PartOf=steamvr.service: it has to outlive the SteamVR restart it may ask for.
+
+[Service]
+# On the host, system Python, standard library only. It stats the XRService log every 2 s and
+# reads only what's new. Notifications go to the Frametop desktop's own D-Bus (found through its
+# plasmashell). With CAMWATCH_AUTO_RESTART=1 in ~/.config/frametop.conf it may restart SteamVR
+# while the headset isn't worn: that also closes the Frametop desktop.
+ExecStart=/usr/bin/python3 @REPO@/hands/ft-camwatch
+Restart=on-failure
+RestartSec=30
+Nice=10
+CPUQuota=5%
+MemoryMax=64M
+
+[Install]
+WantedBy=default.target