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/track/io.h

@@ -45,7 +45,8 @@ private:
 uint64_t mono_ns();
 int64_t raw_minus_mono_ns();   // camera timestamps are CLOCK_MONOTONIC_RAW
 
-// /run/user/UID/frametop, created private to the user if it's missing: where ft-camd's ring
+// /run/user/UID/frametop-hands, created private to the user if it's missing: where ft-camd's ring
 // (FH_RING_NAME) and the hands and gestures files live. Not $XDG_RUNTIME_DIR: a terminal in
-// the Frametop desktop has a private one of its own.
+// the Frametop desktop has a private one of its own. And not /run/user/UID/frametop: that is
+// the desktop session's private runtime folder, which it deletes whenever it starts.
 std::string run_dir();