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>

lib/theme/light.dart

@@ -0,0 +1,63 @@
+// Auto dark/bright from the tablet's light sensor. The Redmi Pad SE has no separate one:
+// Xiaomi derives it from the front camera ("camera_light_Sensor") and offers it as
+// Android's ordinary light sensor, the one auto-brightness reads. No camera permission.
+
+import 'package:flutter/services.dart';
+
+/// Light levels in lux, about once a second, from MainActivity. Ends at once (an error,
+/// which [Themes.followLight] ignores) where there is no sensor or no Android side.
+Stream<double> lightLevels() =>
+    const EventChannel('yukart/light')
+        .receiveBroadcastStream()
+        .map((e) => (e as num).toDouble());
+
+/// Dark or bright from light levels, with a gap and a hold so dusk, shade and
+/// passing lights do not flicker it.
+class LightSwitch {
+  /// Dark at or below this.
+  static const darkAt = 15.0;
+
+  /// Bright at or above this; in between, whatever it was.
+  static const brightAt = 60.0;
+
+  /// How long a level must last before it switches.
+  static const hold = Duration(seconds: 5);
+
+  bool? _dark;
+  DateTime? _since;
+
+  bool? get isDark => _dark;
+
+  /// A change is waiting out its hold.
+  bool get pending => _since != null;
+
+  /// The hold has passed with no reading against it (a sensor that reports only on
+  /// change goes quiet): make the waiting change.
+  void settle() {
+    if (_since == null) return;
+    _dark = !_dark!;
+    _since = null;
+  }
+
+  /// Feed one reading; returns the verdict (null until a reading outside the gap).
+  bool? feed(double lux, DateTime now) {
+    final want = lux <= darkAt
+        ? true
+        : lux >= brightAt
+        ? false
+        : _dark;
+    if (_dark == null) {
+      // The first clear reading decides at once; one in the gap leaves it open.
+      _dark = want;
+    } else if (want == _dark) {
+      _since = null;
+    } else {
+      _since ??= now;
+      if (now.difference(_since!) >= hold) {
+        _dark = want;
+        _since = null;
+      }
+    }
+    return _dark;
+  }
+}