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/autopilot_theme.dart

@@ -0,0 +1,135 @@
+import 'package:flutter/material.dart';
+
+/// Visual theme token set for the AR-Autopilot display app.
+///
+/// Every widget reads design tokens from [AutopilotTheme] via
+/// [AutopilotThemeProvider]. No widget hard-codes colors.
+///
+/// ## Design invariants (apply to ALL themes)
+/// 1. **DISENGAGE** is always the highest-contrast element on screen.
+///    If the palette makes red ambiguous, it changes to amber (wine theme).
+/// 2. **Action buttons** (DODGE, ±1°, ±10°) must be legible at rest —
+///    operators work with gloves in rain; hover/tap is not reliable.
+/// 3. **Compass heading text** uses [textMain] with [accentGlowColor] glow.
+/// 4. **Set-point** always uses [setLight] (amber/gold) — cognitively stable
+///    across palettes; represents "operator intent".
+/// 5. **North mark** is always a warm color ([northColor]) — nautical convention.
+/// 6. **Touch targets**: 48×48 px nominal, 60×60 px for critical controls.
+/// 7. **No glow in light mode**: [accentGlowRadius] == 0.0 for `light` theme.
+class AutopilotTheme {
+  const AutopilotTheme({
+    required this.id,
+    required this.displayName,
+    // Backgrounds
+    required this.background,
+    required this.backgroundMid,
+    required this.backgroundDeep,
+    required this.backgroundDeepest,
+    this.backgroundGradient,
+    // Panels
+    required this.panelBackground,
+    required this.panelBorder,
+    // Text
+    required this.textMain,
+    required this.textMuted,
+    required this.textSoft,
+    required this.textDisabled,
+    // Accent — heading arc, active mode indicator, rudder indicator
+    required this.accentLight,
+    required this.accentMid,
+    required this.accentDark,
+    required this.accentGlowRadius,
+    required this.accentGlowColor,
+    // Set-point / desired heading
+    required this.setLight,
+    required this.setDark,
+    required this.setGlow,
+    // Semantic states
+    required this.okColor,
+    required this.warnColor,
+    required this.northColor,
+    // DISENGAGE — always high contrast, always unambiguous
+    required this.disengageBackground,
+    required this.disengageText,
+    required this.disengageBorder,
+    required this.disengageGlow,
+    // Action buttons (DODGE, ±1°, ±10°)
+    required this.actionButtonBackground,
+    required this.actionButtonBorder,
+    required this.actionButtonText,
+    required this.actionButtonGlow,
+  });
+
+  final String id;
+  final String displayName;
+
+  // ── Backgrounds ────────────────────────────────────────────────────────────
+  /// Dominant background color. Used as solid fill (light) or radial
+  /// gradient center (dark themes). See [backgroundDecoration].
+  final Color background;
+  final Color backgroundMid;
+  final Color backgroundDeep;
+  final Color backgroundDeepest;
+
+  /// Radial gradient for dark themes; `null` for the light theme
+  /// (which uses a solid [background]).
+  final Gradient? backgroundGradient;
+
+  // ── Panels ──────────────────────────────────────────────────────────────────
+  final Gradient panelBackground;
+  final Color panelBorder;
+
+  // ── Text ────────────────────────────────────────────────────────────────────
+  final Color textMain;
+  final Color textMuted;
+  final Color textSoft;
+  final Color textDisabled;
+
+  // ── Accent ──────────────────────────────────────────────────────────────────
+  final Color accentLight;
+  final Color accentMid;
+  final Color accentDark;
+
+  /// Blur radius for accent glow effect. Zero in light mode (no glow).
+  final double accentGlowRadius;
+  final Color accentGlowColor;
+
+  // ── Set-point ───────────────────────────────────────────────────────────────
+  final Color setLight;
+  final Color setDark;
+  final Color setGlow;
+
+  // ── Semantic states ─────────────────────────────────────────────────────────
+  /// GPS OK, NMEA sentence valid, SOG within range.
+  final Color okColor;
+
+  /// Heading error, cross-track error warning.
+  final Color warnColor;
+
+  /// North mark on the compass rose. Always a warm color (never blue).
+  final Color northColor;
+
+  // ── DISENGAGE ───────────────────────────────────────────────────────────────
+  final Gradient disengageBackground;
+  final Color disengageText;
+  final Color disengageBorder;
+  final Color disengageGlow;
+
+  // ── Action buttons (DODGE, ±1°, ±10°) ──────────────────────────────────────
+  final Gradient actionButtonBackground;
+  final Color actionButtonBorder;
+  final Color actionButtonText;
+  final Color actionButtonGlow;
+
+  // ── Computed helpers ────────────────────────────────────────────────────────
+
+  /// [BoxDecoration] for the full-screen background container.
+  BoxDecoration get backgroundDecoration => backgroundGradient != null
+      ? BoxDecoration(gradient: backgroundGradient)
+      : BoxDecoration(color: background);
+
+  /// [BoxShadow] list for accented glow (empty in light mode).
+  List<BoxShadow> glowShadow(Color color, double radius) => radius > 0
+      ? [BoxShadow(color: color, blurRadius: radius, spreadRadius: 1)]
+      : const [];
+}