Eye tracker: one thread for OpenCV and numpy

Nothing called cv2.setNumThreads, so OpenCV kept a pool of one worker per core for
pupil windows of 140 to 240 px. Live, its idle workers spun and yielded about 14,000
times a second each, about a quarter of a core, next to SteamVR's compositor. numpy's
OpenBLAS also started 8 threads that never had work.

eyes_pupil.py now sets OpenCV to one thread, and ft-eyes sets OPENBLAS_NUM_THREADS and
OMP_NUM_THREADS to 1 before numpy loads (a value already in the environment wins).

Replaying fit1 into a scratch share (ft-eyes-replay, 14 s measured, capped at one core
with the replay): threads 13 -> 1, involuntary context switches 3,812/s -> 430/s,
system time 6.9% -> 1.6% of a core. Under that cap the frames it kept up with went from
21-36 to 57-69 a second per eye.

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

gaze/tracker/ft-eyes

@@ -58,7 +58,12 @@ import time
 from collections import deque
 from pathlib import Path
 
-import numpy as np
+# One thread for numpy's BLAS and OpenMP, set before numpy loads: it would start one per core
+# (8 here) for small arrays that never need them. eyes_pupil keeps OpenCV to one as well.
+for _var in ("OPENBLAS_NUM_THREADS", "OMP_NUM_THREADS"):
+    os.environ.setdefault(_var, "1")
+
+import numpy as np  # noqa: E402
 
 sys.path.insert(0, str(Path(__file__).resolve().parent))
 import eyes_model  # noqa: E402