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/themes/ochre_theme.dart

@@ -0,0 +1,66 @@
+import 'package:flutter/material.dart';
+import '../autopilot_theme.dart';
+
+/// **ochre** — Ocre
+///
+/// Classic cabin / aged leather aesthetic. Warm amber-gold accent on deep
+/// brown. DISENGAGE returns to red (which contrasts well against ochre).
+/// okColor is lime-green to stand out from the dominant gold palette.
+const AutopilotTheme ochreTheme = AutopilotTheme(
+  id: 'ochre',
+  displayName: 'Ocre',
+
+  background: Color(0xFF2A1A08),
+  backgroundMid: Color(0xFF2A1A08),
+  backgroundDeep: Color(0xFF1A1004),
+  backgroundDeepest: Color(0xFF080502),
+  backgroundGradient: RadialGradient(
+    colors: [Color(0xFF2A1A08), Color(0xFF0F0904)],
+    stops: [0.0, 0.7],
+  ),
+
+  panelBackground: LinearGradient(
+    begin: Alignment.topCenter,
+    end: Alignment.bottomCenter,
+    colors: [Color(0xEB32200C), Color(0xEB181008)], // rgba(50,32,12,0.92) → rgba(24,16,8,0.92)
+  ),
+  panelBorder: Color(0x52D97706), // rgba(217,119,6,0.32)
+
+  textMain: Color(0xFFFEF3C7),
+  textMuted: Color(0xFFA8855A),
+  textSoft: Color(0xFFFCD34D),
+  textDisabled: Color(0xFF6B5435),
+
+  accentLight: Color(0xFFFBBF24),
+  accentMid: Color(0xFFD97706),
+  accentDark: Color(0xFF92400E),
+  accentGlowRadius: 18.0,
+  accentGlowColor: Color(0xB3D97706),
+
+  setLight: Color(0xFFFDE68A),
+  setDark: Color(0xFFB45309),
+  setGlow: Color(0x66FDE68A),
+
+  okColor: Color(0xFF84CC16),  // lime-green — contrasts with gold
+  warnColor: Color(0xFFFDE68A),
+  northColor: Color(0xFFFDE047),
+
+  // DISENGAGE: red — contrasts well on ochre background
+  disengageBackground: LinearGradient(
+    begin: Alignment.topCenter,
+    end: Alignment.bottomCenter,
+    colors: [Color(0xFFDC2626), Color(0xFF7F1D1D)],
+  ),
+  disengageText: Color(0xFFFFFFFF),
+  disengageBorder: Color(0xFFF87171),
+  disengageGlow: Color(0x99DC2626),
+
+  actionButtonBackground: LinearGradient(
+    begin: Alignment.topCenter,
+    end: Alignment.bottomCenter,
+    colors: [Color(0xFF7A4E1A), Color(0xFF3A240C)],
+  ),
+  actionButtonBorder: Color(0xFFFBBF24),
+  actionButtonText: Color(0xFFFEF3C7),
+  actionButtonGlow: Color(0x66FBBF24),
+);