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>

docs/reference.md

@@ -173,7 +173,7 @@ power/run.sh log
 
 Your hands show over the screens: where a tracked hand is between an eye and a screen, ft-screens lets that eye see the room through the screen. The same tracker also detects pinches, for clicking where you look with the gaze pointer (not wired to the pointer yet). It's optional: `hands/run.sh install`, or the last step of `install.sh`.
 
-- `ft-camd` borrows XRService's camera buffers and publishes the four IR tracking cameras to `/run/user/UID/frametop/cam-ring`. It runs on the host as `frametop-camd.service`, with file capabilities that `hands/run.sh install` sets through sudo, and it drops them once set up. A rebuild clears them: `hands/run.sh caps`.
+- `ft-camd` borrows XRService's camera buffers and publishes the four IR tracking cameras to `/run/user/UID/frametop-hands/cam-ring`. It runs on the host as `frametop-camd.service`, with file capabilities that `hands/run.sh install` sets through sudo, and it drops them once set up. A rebuild clears them: `hands/run.sh caps`.
 - `ft-hands` runs in the `dev` container as `frametop-hands.service`. It finds and triangulates the hands, and publishes `hands` (read by ft-screens' cutouts) and `gestures` (pinches) next to the ring.
 - Both start and stop with SteamVR. `hands/run.sh status` and `hands/run.sh log` show how they're doing.
 - Settings in `~/.config/frametop.conf`: `HANDS_SWAP_SIDES` (after some SteamVR restarts the side cameras' names come out swapped, and hands land beside the holes; `hands/tools/check_sides.py --ring` tells) and `HANDS_CPUS`.