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/rudder_indicator.dart

@@ -0,0 +1,125 @@
+import 'package:flutter/material.dart';
+import 'package:provider/provider.dart';
+
+import '../../theme/autopilot_theme.dart';
+import '../../theme/theme_provider.dart';
+
+/// Rudder angle indicator — horizontal bar from –35° (port) to +35° (starboard).
+///
+/// [rudderDeg] is the actual rudder angle in degrees.
+///   Negative = port, positive = starboard.
+/// [limitDeg] is the hard stop (default 35°).
+class RudderIndicator extends StatelessWidget {
+  const RudderIndicator({
+    super.key,
+    required this.rudderDeg,
+    this.limitDeg = 35.0,
+    this.height = 48,
+  });
+
+  final double rudderDeg;
+  final double limitDeg;
+  final double height;
+
+  @override
+  Widget build(BuildContext context) {
+    final theme = context.watch<AutopilotThemeProvider>().current;
+    return SizedBox(
+      height: height,
+      child: CustomPaint(
+        painter: _RudderPainter(
+          theme: theme,
+          rudderDeg: rudderDeg,
+          limitDeg: limitDeg,
+        ),
+      ),
+    );
+  }
+}
+
+class _RudderPainter extends CustomPainter {
+  const _RudderPainter({
+    required this.theme,
+    required this.rudderDeg,
+    required this.limitDeg,
+  });
+
+  final AutopilotTheme theme;
+  final double rudderDeg;
+  final double limitDeg;
+
+  @override
+  void paint(Canvas canvas, Size size) {
+    final t = theme;
+    final cx = size.width / 2;
+    final cy = size.height / 2;
+    final halfW = size.width / 2 - 12;
+
+    // Track
+    canvas.drawRRect(
+      RRect.fromRectAndRadius(
+        Rect.fromCenter(center: Offset(cx, cy), width: size.width - 24, height: 6),
+        const Radius.circular(3),
+      ),
+      Paint()..color = t.panelBorder,
+    );
+
+    // Centre line
+    canvas.drawLine(
+      Offset(cx, cy - 10),
+      Offset(cx, cy + 10),
+      Paint()
+        ..color = t.textMuted
+        ..strokeWidth = 1,
+    );
+
+    // Rudder fill
+    final fraction = (rudderDeg / limitDeg).clamp(-1.0, 1.0);
+    final fillWidth = (halfW * fraction.abs()).clamp(0.0, halfW);
+    if (fillWidth > 1) {
+      final left = fraction < 0 ? cx - fillWidth : cx;
+      canvas.drawRRect(
+        RRect.fromRectAndRadius(
+          Rect.fromLTWH(left, cy - 3, fillWidth, 6),
+          const Radius.circular(2),
+        ),
+        Paint()..color = t.accentMid,
+      );
+    }
+
+    // Indicator knob
+    final kx = cx + halfW * fraction;
+    canvas.drawCircle(
+      Offset(kx, cy),
+      9,
+      Paint()..color = t.accentLight,
+    );
+    if (t.accentGlowRadius > 0) {
+      canvas.drawCircle(
+        Offset(kx, cy),
+        9,
+        Paint()
+          ..color = t.accentGlowColor
+          ..maskFilter = MaskFilter.blur(BlurStyle.normal, t.accentGlowRadius / 2),
+      );
+    }
+
+    // Labels
+    final labelStyle = TextStyle(color: t.textMuted, fontSize: 10);
+    _drawLabel(canvas, 'P', Offset(12, cy), labelStyle);
+    _drawLabel(canvas, 'S', Offset(size.width - 12, cy), labelStyle);
+  }
+
+  void _drawLabel(Canvas canvas, String text, Offset center, TextStyle style) {
+    final span = TextSpan(text: text, style: style);
+    final tp = TextPainter(
+      text: span,
+      textDirection: TextDirection.ltr,
+    )..layout();
+    tp.paint(canvas, center - Offset(tp.width / 2, tp.height / 2));
+  }
+
+  @override
+  bool shouldRepaint(_RudderPainter old) =>
+      old.rudderDeg != rudderDeg || old.theme != theme;
+}