Gaze first: abandoned; keep the controller experiment for reference

The last state of the experiment, kept on this branch only:
- input/gazefirst.py, input/steamui.py: the relay reading the controllers from
  vrserver's web socket, switching the compositor binding, and driving the Steam
  UI filter (whose block now expires unless renewed).
- The helper's trigger and bumper holds (steered by the controller's position),
  the laser keeper, and the click gate that waits for the laser.
- pointer/probe/focustest (overlay flag 1 << 4, as an input client too),
  vrsetting (SteamVR settings through vrserver), lasertest --reclaim.

Every press and release Steam sees takes SteamVR out of laser mode, and taking
the laser back breaks clicks and drags. See docs/gaze-controllers.md on
experimental.

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

pointer/probe/vrsetting.cpp

@@ -0,0 +1,48 @@
+// Read or change one SteamVR setting through vrserver (IVRSettings). Editing steamvr.vrsettings
+// while SteamVR runs doesn't stick: vrserver writes its own copy back on the next change.
+// Runs as a background OpenVR client (in the dev container).
+// Usage: vrsetting get SECTION KEY
+//        vrsetting set-bool SECTION KEY true|false
+//        vrsetting remove SECTION KEY
+#include <openvr.h>
+
+#include <cstdio>
+#include <cstring>
+
+int main(int argc, char **argv) {
+    if (argc < 4 || (std::strcmp(argv[1], "set-bool") == 0 && argc < 5)) {
+        std::fprintf(stderr, "usage: %s get|set-bool|remove SECTION KEY [true|false]\n", argv[0]);
+        return 2;
+    }
+    const char *cmd = argv[1], *section = argv[2], *key = argv[3];
+    vr::EVRInitError err = vr::VRInitError_None;
+    vr::VR_Init(&err, vr::VRApplication_Background);
+    if (err != vr::VRInitError_None) {
+        std::printf("VR_Init failed: %s\n", vr::VR_GetVRInitErrorAsEnglishDescription(err));
+        return 1;
+    }
+    vr::EVRSettingsError serr = vr::VRSettingsError_None;
+    int rc = 0;
+    if (!std::strcmp(cmd, "get")) {
+        char s[512] = "";
+        vr::VRSettings()->GetString(section, key, s, sizeof s, &serr);
+        if (serr == vr::VRSettingsError_None) std::printf("%s\n", s);
+        else {
+            const bool b = vr::VRSettings()->GetBool(section, key, &serr);
+            std::printf("%s\n", serr == vr::VRSettingsError_None ? (b ? "true" : "false") : "(unset)");
+        }
+    } else if (!std::strcmp(cmd, "set-bool")) {
+        vr::VRSettings()->SetBool(section, key, !std::strcmp(argv[4], "true"), &serr);
+    } else if (!std::strcmp(cmd, "remove")) {
+        vr::VRSettings()->RemoveKeyInSection(section, key, &serr);
+    } else {
+        std::fprintf(stderr, "unknown command %s\n", cmd);
+        rc = 2;
+    }
+    if (serr != vr::VRSettingsError_None) {
+        std::printf("error: %s\n", vr::VRSettings()->GetSettingsErrorNameFromEnum(serr));
+        rc = 1;
+    }
+    vr::VR_Shutdown();
+    return rc;
+}