Hands: fixes from the first headset test

- The runtime files move to /run/user/UID/frametop-hands/: the desktop
  session deletes /run/user/UID/frametop at every start.
- The cutout copy shader runs at highp: mediump (16-bit on Adreno)
  stepped 1.7 texels across a 3440-pixel screen.
- One hand no longer pinches both sides after its left/right call flips
  mid-pinch, and --pinch-palm-down (0.6) holds back pinches with the palm
  facing down (typing on a lap keyboard).
- ft-camd judges a colour frame fresh by its luma rows only, and logs
  per-buffer changes at stale colour frames with FT_CAMD_DEBUG=1.
- hands/run.sh caps skips the setcap when ft-camd already has them.
- The replay tool dumps poses (--poses), and its pinch events carry the
  hand id.

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

hands/frametop-hands.service

@@ -8,7 +8,7 @@ PartOf=steamvr.service
 
 [Service]
 # In the dev container (it's built against Fedora's libraries). It reads ft-camd's ring and
-# writes /run/user/UID/frametop/hands and gestures. Settings: HANDS_* in ~/.config/frametop.conf.
+# writes /run/user/UID/frametop-hands/hands and gestures. Settings: HANDS_* in ~/.config/frametop.conf.
 ExecStartPre=-@REPO@/scripts/container-up.sh
 ExecStartPre=-/usr/bin/pkill -x ft-hands
 ExecStart=%h/.local/bin/distrobox enter dev -- @REPO@/hands/build/ft-hands --status 60