Gaze: ft-gaze prints and reads only the sources in use

ft-gaze computed and printed all six sources for every sample: about 1.3 KB of JSON a line
with our tracker (practice2), 120 KB a second through podman's stdio relay for ft-gazed to
json.loads 90 times a second. It also read SteamVR's gaze action for every sample outside
games (UpdateActionState and GetEyeTrackingDataRelativeToNow, two calls into vrserver, 180 a
second), though with our tracker ft-gazed only uses own and mmap1.

- ft-gaze takes --sources LIST (action, mmap1, mmap2, left, right, own, and eye for the EYE
  object; all by default, so the probe and ft-eyes-session are unchanged), and with
  --watch-stdin a line "sources LIST" on stdin switches them. A source left out isn't read
  and prints as {"ok":0} ("eye" as null), so every line keeps the same keys. An older
  ft-gaze ignores both, and prints everything as before.
- ft-gazed asks for what it reads: own,mmap1 with our tracker; left,right,mmap1 with
  SteamVR's eyes; the source plus mmap1 and mmap2 on the older one-source path. While a
  check or the calibration runs or waits to open, all of them, since checks record every
  source (the calibration fits the action's correction too) and the fit check reads "eye".
  It switches as soon as that changes, well inside the check's 0.45 s settle.
- So the action is read only during checks, or with --source action.

On recorded samples, a line with own and mmap1 is about 700 bytes instead of 1,300
(practice2), and one with left, right and mmap1 about 550 instead of 940 (test1).
gaze/test/idle-test.py now checks that ft-gaze starts with every source for a check and is
then switched to those in use, without the action or own; all its checks pass.

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

gaze/README.md

@@ -2,7 +2,7 @@
 
 The Steam Frame's eye tracking as pointer input: a gaze mode for the 3D mouse (the pointer goes where you look, and the mouse does the last bit), and the tools to calibrate and measure it.
 
-- `ft-gaze` (C++, OpenVR, runs in the dev container) reads the eye tracker and prints one JSON line per sample (90 Hz). For each source, it gives the gaze direction relative to the head and the Frametop screen pixel it lands on.
+- `ft-gaze` (C++, OpenVR, runs in the dev container) reads the eye tracker and prints one JSON line per sample (90 Hz). For each source, it gives the gaze direction relative to the head and the Frametop screen pixel it lands on. The gaze service asks it for only the sources it uses (`--sources`, and `sources LIST` on its stdin): our tracker and mmap set 1 with Own tracker, about 700 bytes a line instead of 1.3 KB, and every source while a check or the calibration runs. So SteamVR's gaze action, the only source that calls into vrserver (twice a sample), is read only then. The probe gets them all.
 - `gazecal.py` has what the probe and the gaze service share: the correction models, filters, and the reader for SteamVR's eye tracking log.
 - `tracker/` is our own eye tracker, an alternative to SteamVR's: `ft-eyes` finds the pupils and glints in the eye-camera frames that `ft-eyegrab` (a small root service) copies out of SteamVR's tracker. See "Our own eye tracker" below.
 - `probe/ft-gazeprobe` (GTK 4, host Python) is a fullscreen playground, for developing the gaze tracking: day to day, the calibration and the checks run in the headset panel (Quick check, Calibrate, and Check headset fit on the Gaze page). It runs ft-gaze, draws where you're looking, measures accuracy, and tries out hold-to-adjust clicking with a calibration that learns from your adjustments.