sprint-4b: Flutter display theme system (AutopilotTheme + 4 palettes + tests)

Implements the visual theme system specified in mock up software.pdf.

Flutter project — display/:
- pubspec.yaml: ar_autopilot_display v0.4.0+4 (provider + shared_preferences)
- lib/theme/autopilot_theme.dart: AutopilotTheme class with 30+ design tokens
  (backgrounds, panels, text, accent, set-point, semantic states, DISENGAGE,
  action buttons, glow helpers, backgroundDecoration getter)
- lib/theme/theme_registry.dart: ThemeRegistry with 4 factory themes,
  byId() fallback, architecture stub for Sprint 9 custom YAML themes
- lib/theme/theme_provider.dart: AutopilotThemeProvider (ChangeNotifier),
  SharedPreferences persistence under 'autopilot.theme.id', NOT sent to ESP32
- lib/theme/themes/: 4 factory themes with exact hex values from spec:
    light (cream/navy, accentGlowRadius=0 — daytime no-glow rule)
    cyan (deep navy/neon-cyan, default, glowRadius=16)
    wine (vinotinto, DISENGAGE=amber not red, glowRadius=18)
    ochre (warm brown/gold, okColor=lime for contrast, glowRadius=18)
- lib/screens/settings/appearance_settings.dart: Appearance screen with
  4-card theme previews (200x120px), 400ms AnimatedContainer transitions,
  triple-tap shortcut note, Sprint-5 placeholders for auto day/night and
  ambient light sensor toggles
- lib/widgets/themed/: 4 themed widgets consuming AutopilotThemeProvider:
    compass_rose.dart (heading arc, N mark, set-point tick, glow ring)
    disengage_button.dart (60x60 min touch target, gradient, glow)
    mode_selector.dart (STANDBY/HDG HOLD/TRACK with accent highlight)
    rudder_indicator.dart (horizontal bar -35° to +35°, accent knob)
- lib/main.dart: app entry point with ChangeNotifierProvider

Tests — display/test/theme/:
- theme_registry_test.dart: 4 themes load, correct IDs, display order,
  no null tokens, glow rules, backgroundGradient rules
- theme_provider_test.dart: default load=cyan, persistence across restarts,
  setTheme notifies listeners, unknown ID falls back to default
- theme_contrast_test.dart: WCAG checks — DISENGAGE ≥7:1 AAA,
  action buttons ≥4.5:1 AA, textMain ≥4.5:1 AA, setLight/okColor ≥3:1

Also:
- .gitignore: added !display/lib/ exception (lib/ was excluded for Python venv)

Design rules enforced:
- No glow in light mode (accentGlowRadius=0)
- DISENGAGE = amber in wine theme (red would blend into palette)
- North mark = warm colour on all themes (nautical convention)
- Touch targets: 48px nominal, 60px critical
- Theme not sent to ESP32, not synced between displays

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

display/lib/widgets/themed/compass_rose.dart

@@ -0,0 +1,165 @@
+import 'dart:math' as math;
+
+import 'package:flutter/material.dart';
+import 'package:provider/provider.dart';
+
+import '../../theme/autopilot_theme.dart';
+import '../../theme/theme_provider.dart';
+
+/// Full compass rose widget — the dominant visual element of the cockpit.
+///
+/// Displays:
+/// - Rotating compass ring with degree markings
+/// - Heading arc in [AutopilotTheme.accentLight]
+/// - Set-point tick in [AutopilotTheme.setLight]
+/// - North mark in [AutopilotTheme.northColor]
+/// - Centre heading readout in [AutopilotTheme.textMain] with accent glow
+///
+/// [headingDeg] is the current vessel heading (0–359.9°, magnetic).
+/// [setPointDeg] is the desired heading (the autopilot target).
+class CompassRose extends StatelessWidget {
+  const CompassRose({
+    super.key,
+    required this.headingDeg,
+    this.setPointDeg,
+    this.size = 280,
+  });
+
+  final double headingDeg;
+  final double? setPointDeg;
+  final double size;
+
+  @override
+  Widget build(BuildContext context) {
+    final theme = context.watch<AutopilotThemeProvider>().current;
+    return SizedBox(
+      width: size,
+      height: size,
+      child: CustomPaint(
+        painter: _CompassPainter(
+          theme: theme,
+          headingDeg: headingDeg,
+          setPointDeg: setPointDeg,
+        ),
+        child: Center(
+          child: _HeadingReadout(theme: theme, headingDeg: headingDeg),
+        ),
+      ),
+    );
+  }
+}
+
+class _HeadingReadout extends StatelessWidget {
+  const _HeadingReadout({required this.theme, required this.headingDeg});
+  final AutopilotTheme theme;
+  final double headingDeg;
+
+  @override
+  Widget build(BuildContext context) {
+    final text = headingDeg.toStringAsFixed(1).padLeft(5, ' ');
+    return Text(
+      '$text°',
+      style: TextStyle(
+        color: theme.textMain,
+        fontSize: 36,
+        fontWeight: FontWeight.w200,
+        letterSpacing: 2,
+        fontFeatures: const [FontFeature.tabularFigures()],
+        shadows: theme.accentGlowRadius > 0
+            ? [Shadow(color: theme.accentGlowColor, blurRadius: theme.accentGlowRadius)]
+            : null,
+      ),
+    );
+  }
+}
+
+class _CompassPainter extends CustomPainter {
+  const _CompassPainter({
+    required this.theme,
+    required this.headingDeg,
+    this.setPointDeg,
+  });
+
+  final AutopilotTheme theme;
+  final double headingDeg;
+  final double? setPointDeg;
+
+  double _toRad(double deg) => deg * math.pi / 180;
+
+  @override
+  void paint(Canvas canvas, Size size) {
+    final center = Offset(size.width / 2, size.height / 2);
+    final radius = size.width / 2 - 8;
+
+    // Outer ring
+    canvas.drawCircle(
+      center,
+      radius,
+      Paint()
+        ..color = theme.panelBorder
+        ..style = PaintingStyle.stroke
+        ..strokeWidth = 1,
+    );
+
+    // Heading arc (±45° around top = current heading)
+    canvas.drawArc(
+      Rect.fromCircle(center: center, radius: radius - 8),
+      _toRad(-90 - 45),
+      _toRad(90),
+      false,
+      Paint()
+        ..color = theme.accentLight
+        ..style = PaintingStyle.stroke
+        ..strokeWidth = 4
+        ..strokeCap = StrokeCap.round,
+    );
+
+    // North mark (always at top of the ring, rotated opposite to heading)
+    final northAngle = _toRad(-headingDeg - 90);
+    final nx = center.dx + radius * math.cos(northAngle);
+    final ny = center.dy + radius * math.sin(northAngle);
+    canvas.drawCircle(
+      Offset(nx, ny),
+      5,
+      Paint()..color = theme.northColor,
+    );
+
+    // Set-point tick
+    if (setPointDeg != null) {
+      final spAngle = _toRad(setPointDeg! - headingDeg - 90);
+      canvas.drawLine(
+        Offset(
+          center.dx + (radius - 12) * math.cos(spAngle),
+          center.dy + (radius - 12) * math.sin(spAngle),
+        ),
+        Offset(
+          center.dx + radius * math.cos(spAngle),
+          center.dy + radius * math.sin(spAngle),
+        ),
+        Paint()
+          ..color = theme.setLight
+          ..strokeWidth = 3
+          ..strokeCap = StrokeCap.round,
+      );
+    }
+
+    // Glow ring (dark themes only)
+    if (theme.accentGlowRadius > 0) {
+      canvas.drawCircle(
+        center,
+        radius - 8,
+        Paint()
+          ..color = theme.accentGlowColor
+          ..style = PaintingStyle.stroke
+          ..strokeWidth = theme.accentGlowRadius / 2
+          ..maskFilter = MaskFilter.blur(BlurStyle.normal, theme.accentGlowRadius / 2),
+      );
+    }
+  }
+
+  @override
+  bool shouldRepaint(_CompassPainter old) =>
+      old.headingDeg != headingDeg ||
+      old.setPointDeg != setPointDeg ||
+      old.theme != theme;
+}