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/camd/fhring.h

@@ -1,7 +1,7 @@
 /*
  * fhring - the shared-memory frame ring ft-camd writes and trackers read.
  *
- * One file, /run/user/UID/frametop/cam-ring (FH_RING_NAME in the user's runtime
+ * One file, /run/user/UID/frametop-hands/cam-ring (FH_RING_NAME in the user's runtime
  * folder; the folder is private to the user), holds a header, then for each camera
  * a few slots, each a slot header followed by the image rows packed tightly
  * (stride == width for 8-bit mono). Only complete, bright frames are published.
@@ -25,7 +25,7 @@
 #define FH_RING_VERSION   1
 #define FH_RING_MAX_CAMS  8
 #define FH_RING_SLOTS     4
-#define FH_RING_NAME      "frametop/cam-ring"    /* in /run/user/UID */
+#define FH_RING_NAME      "frametop-hands/cam-ring"    /* in /run/user/UID */
 
 enum {
     FH_FMT_GREY8 = 0,