Auto dark/bright takes a light sensor where the device allows one

lib/theme/light.dart: dark at 15 lux or less, bright from 60, held 5 s; Settings shows the
reading. The Redmi Pad SE refuses apps its only light sensor (Xiaomi's, from the front
camera), so there Auto keeps following Android's dark theme. DASH-NEXT.md records the
other lux sources.

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

docs/impl/plans/DASH-NEXT.md

@@ -74,7 +74,17 @@ chosen readouts.
 **Done (2026-10-15):** Settings has Auto / Dark / Bright (a segmented control, kept across
 restarts). Auto follows Android's dark theme (`platformBrightness`, so its schedule too). The
 status bar's button (it moved there with item 1) holds the other variant, and a long press
-returns it to auto. Not done: the light sensor as an auto source.
+returns it to auto.
+
+**Light sensor, tried (2026-10-15):** Auto takes a light sensor over Android's dark theme
+where the device lets an app read one (`theme/light.dart`: dark at ≤ 15 lux, bright from 60,
+held 5 s; Settings shows the reading). **The Redmi Pad SE does not:** its only light sensor,
+`camera_light_Sensor`, is derived by Xiaomi from the front camera, and `registerListener` refuses
+apps at every rate (only the system's auto-brightness reads it). Its automatic brightness is not
+exposed either (`screen_brightness` is the manual level). Other lux sources, if wanted:
+the Orin's camera (its exposure and gain say how bright the road is; svc-vision would publish
+it), opening the tablet's front camera ourselves (camera permission, the in-use dot, and the
+camera's power), or sunrise and sunset from GNSS position and time (no sensor; tunnels ignored).
 What was asked:
 
 Today the variant is a switch (the Dash's top-right button, and Settings). Wanted: