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/theme/theme_registry.dart

@@ -0,0 +1,44 @@
+import 'autopilot_theme.dart';
+import 'themes/light_theme.dart';
+import 'themes/cyan_theme.dart';
+import 'themes/wine_theme.dart';
+import 'themes/ochre_theme.dart';
+
+/// Central registry of all factory [AutopilotTheme]s.
+///
+/// Themes are keyed by their [AutopilotTheme.id] string.
+///
+/// ## Display order
+/// `light → cyan → wine → ochre` (shown in the Appearance selector grid).
+///
+/// ## Custom themes (Sprint 9 / Tier 3)
+/// Architecture is ready for integrators to ship custom YAML themes in
+/// `/config/themes/*.yaml`. [ThemeRegistry] will merge them with the
+/// factory set at startup. Not active until Sprint 9.
+abstract final class ThemeRegistry {
+  /// Theme id used when no preference is stored (first run).
+  static const String defaultId = 'cyan';
+
+  static final Map<String, AutopilotTheme> _registry = {
+    lightTheme.id: lightTheme,
+    cyanTheme.id: cyanTheme,
+    wineTheme.id: wineTheme,
+    ochreTheme.id: ochreTheme,
+  };
+
+  /// All available themes in UI display order.
+  static List<AutopilotTheme> get all => const [
+        lightTheme,
+        cyanTheme,
+        wineTheme,
+        ochreTheme,
+      ];
+
+  /// Returns the theme for [id].
+  /// Falls back to [defaultId] (cyan) if [id] is not registered.
+  static AutopilotTheme byId(String id) =>
+      _registry[id] ?? _registry[defaultId]!;
+
+  /// Whether [id] belongs to a registered theme.
+  static bool contains(String id) => _registry.containsKey(id);
+}