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>

screens/handcut.cpp

@@ -59,7 +59,7 @@ bool Hands::Read() {
         const int64_t now = MonoNs();
         if (now - lastOpenTry_ < 1'000'000'000) return false;
         lastOpenTry_ = now;
-        const std::string path = "/run/user/" + std::to_string(getuid()) + "/frametop/hands";
+        const std::string path = "/run/user/" + std::to_string(getuid()) + "/frametop-hands/hands";
         fd_ = open(path.c_str(), O_RDONLY | O_CLOEXEC | O_NOFOLLOW);
         if (fd_ < 0) return false;
         struct stat st;
@@ -295,7 +295,7 @@ void main() {
 // The client's pixels, opaque (its alpha is ignored, as IgnoreTextureAlpha did).
 const char *kCopy = R"(
 #extension GL_OES_EGL_image_external : require
-precision mediump float;
+precision highp float;   // mediump (16-bit on Adreno) steps 1.7 texels across a 3440-pixel screen
 uniform samplerExternalOES tex;
 varying vec2 uv;
 void main() { gl_FragColor = vec4(texture2D(tex, uv).rgb, 1.0); })";

screens/handcut.h

@@ -2,7 +2,7 @@
 // room (Room View) through the screen instead of the screen drawn over the hand.
 //
 // ft-hands (hands/) publishes the hands it sees with the headset's cameras to
-// /run/user/UID/frametop/hands (hands/include/fh_hands.h):
+// /run/user/UID/frametop-hands/hands (hands/include/fh_hands.h):
 // capsules (finger bones, palm, forearm) in the head frame at capture time. Hands turns
 // them into the room with the head pose at that time. Project() finds where each eye
 // sees them on a panel, and Renderer draws the panel's client buffer into a side-by-side

screens/handtest.cpp

@@ -4,7 +4,7 @@
 //
 //   ft-handtest [--distance m] [--width m] [--seconds s]
 //
-// Needs hand tracking running (hands/run.sh; ft-hands publishes /run/user/UID/frametop/hands).
+// Needs hand tracking running (hands/run.sh; ft-hands publishes /run/user/UID/frametop-hands/hands).
 // Build: screens/build.sh (build/ft-handtest), run in the dev container.
 #include "handcut.h"