Gaze: ft-eyes and ft-gaze below SteamVR's priority

ft-eyes and ft-gaze run in the dev container through distrobox, so they live in podman's
libpod scope: frametop-gaze.service's limits never reach them, and they ran at nice 0
next to vrcompositor and vrserver, also at nice 0.

- ft-eyes sets itself to nice 10 and SCHED_BATCH at start, before its threads. Batch
  turns off wakeup preemption, so a frame ft-eyes wakes up for can wait out a running
  compositor's turn; a few ms late costs the gaze little.
- ft-gaze sets nice 5 before its threads start, but stays SCHED_OTHER: each sample goes
  on to the pointer, and batch would add the same wait to every one of them.
- Both only ever lower their priority (a higher nice already set wins), and a failure
  is logged and ignored. Checked in the dev container: nice 0 -> 10, policy 0 -> 3
  (SCHED_BATCH) without any capability.

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

gaze/tracker/ft-eyes

@@ -377,7 +377,24 @@ def serve(sock, tracker):
                 pass
 
 
+def below_steamvr():
+    """Nice 10 and SCHED_BATCH, for this thread and those it starts. ft-eyes runs in the dev
+    container's podman scope, where the gaze service's unit doesn't reach it, so it ran at
+    nice 0 on the cores vrcompositor and vrserver use. Batch also lets a waking ft-eyes wait
+    for the running task's turn instead of taking the core: a frame a few ms late costs the
+    gaze little, a late compositor frame costs a dropped frame in the headset."""
+    try:
+        os.setpriority(os.PRIO_PROCESS, 0, max(os.getpriority(os.PRIO_PROCESS, 0), 10))
+    except OSError as e:
+        print(f"ft-eyes: nice: {e}", file=sys.stderr, flush=True)
+    try:
+        os.sched_setscheduler(0, os.SCHED_BATCH, os.sched_param(0))
+    except (OSError, AttributeError) as e:
+        print(f"ft-eyes: SCHED_BATCH: {e}", file=sys.stderr, flush=True)
+
+
 def main():
+    below_steamvr()
     verbose = "-v" in sys.argv
     if "--watch-stdin" in sys.argv:
         # Run by ft-gazed through distrobox, which doesn't pass a stop on: quit when our