Let the gaze pointer use our own eye tracker, and weight the eyes

Frametop Input Settings' Gaze page gets two settings, saved in frametop.conf and
read again by ft-gazed when the file changes: Eye tracker (GAZE_TRACKER: SteamVR's
or our own, frame-eyes' fe-trackd) and Eye bias (GAZE_EYE: auto, left, right).

ft-gazed now combines the eyes, each calibrated on its own: SteamVR's set 2 eyes
with the probe's Left eye and Right eye calibrations, or our tracker's eyes as
they come. Without per-eye calibrations, or with --source, it keeps the older
one-source path. A pointer nudge finds its look from the raw gaze the helper
echoes back; with our tracker it goes to fe-trackd as a click.

The bias leans instead of choosing (gazecal.EyeWeights): on 306 live clicks the
eyes' errors partly cancelled, both together 0.65 deg off against 0.96 and 1.11
for either alone. Left or Right counts that eye twice; auto weights each eye by
its RMS miss at its last 20 nudges, since the calibration's fit picked the wrong
eye on SteamVR's test.

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

session/frametop.conf.example

@@ -30,6 +30,8 @@ POINTER_GAZE_RETAKE=5      # gaze mode: how far (degrees) you look away from the
 POINTER_GAZE_NUDGE_MAX=8   # gaze mode: a mouse nudge up to this far (degrees) before a click is learned as the eye tracker's error
 POINTER_GAZE_HOLD=0.5      # gaze mode: a press held this long (s) without moving becomes a real press (to drag); moved or released sooner, it clicks where you let go
 POINTER_GAZE_SHOW=1        # gaze mode: the dot shows this long (s) after the mouse moves it; also while a press is held, and a pulse per click
+GAZE_TRACKER=steam         # gaze service: steam = SteamVR's eye tracker | own = our own (frame-eyes' fe-trackd, calibrated in the gaze probe with Own tracker)
+GAZE_EYE=auto              # gaze service: eye bias. auto = each eye weighted by how far off it was at your recent nudges | left | right = that eye counts twice
 META_DASHBOARD=0   # 1 = a Meta tap on a pass-through keyboard toggles the SteamVR dashboard (pointer mode only)
 SHARE_KEYS=0       # 1 = keys of keyboards grabbed for the desktop also go to @frametop_keys, for hotkey tools (any local process can listen)
 DISPLAY_OFF_MIN=0  # minutes without use before frametop-power turns the displays off, even if the headset seems worn (0 = never)