feat(display): Sprint 4 — pantalla principal del autopilot

Archivos nuevos:
  display/lib/data/autopilot_state.dart
    - ChangeNotifier con todos los datos del autopilot
    - Simulación de velero en mar (heading drift / P-controller)
    - API pública estable: engage(), disengage(), adjustSetpoint(), selectMode()
    - Sprint 7: los internos se reemplazan por Modbus RTU, la API no cambia

  display/lib/screens/cockpit/cockpit_screen.dart
    - Pantalla principal: TopBar, ModeSelector, CompassRose, DataStrip,
      HeadingAdjustBar, RudderIndicator, ENGAGE/DISENGAGE
    - Logo con triple-tap para ciclar temas (StateWidget con Timer)
    - Indicador DEMO visible cuando isConnected == false
    - Engranaje → AppearanceSettingsScreen

  display/lib/widgets/themed/engage_button.dart
    - Botón verde con glow; dimmed cuando ya está engaged

  display/lib/widgets/themed/heading_adjust_bar.dart
    - Botones << < [SET 048.0°] > >>
    - Deshabilitado cuando mode == STANDBY

  display/lib/widgets/themed/status_chip.dart
    - Indicador de punto + label para NMEA / GPS (ok / warn / off)

Modificado:
  display/lib/main.dart
    - MultiProvider: agrega AutopilotState al árbol de providers
    - Ruta inicial: CockpitScreen.routeName ('/') en lugar de Appearance

AR_electronics — AR-Autopilot Project

display/lib/widgets/themed/engage_button.dart

@@ -0,0 +1,95 @@
+import 'package:flutter/material.dart';
+import 'package:provider/provider.dart';
+
+import '../../theme/autopilot_theme.dart';
+import '../../theme/theme_provider.dart';
+
+/// ENGAGE button — activates heading-hold at the current vessel heading.
+///
+/// Shown enabled (green, glowing) only when autopilot is in STANDBY.
+/// When already engaged, [enabled] is false and the button dims.
+///
+/// Minimum touch target: 60×60 px (critical control, see design invariant §6).
+class EngageButton extends StatefulWidget {
+  const EngageButton({
+    super.key,
+    required this.onPressed,
+    this.enabled = true,
+  });
+
+  final VoidCallback? onPressed;
+  final bool enabled;
+
+  @override
+  State<EngageButton> createState() => _EngageButtonState();
+}
+
+class _EngageButtonState extends State<EngageButton> {
+  bool _pressed = false;
+
+  @override
+  Widget build(BuildContext context) {
+    final theme = context.watch<AutopilotThemeProvider>().current;
+    final enabled = widget.enabled && widget.onPressed != null;
+
+    return GestureDetector(
+      onTapDown: enabled ? (_) => setState(() => _pressed = true) : null,
+      onTapUp: enabled
+          ? (_) {
+              setState(() => _pressed = false);
+              widget.onPressed!();
+            }
+          : null,
+      onTapCancel: () => setState(() => _pressed = false),
+      child: AnimatedContainer(
+        duration: const Duration(milliseconds: 120),
+        constraints: const BoxConstraints(minWidth: 60, minHeight: 60),
+        padding: const EdgeInsets.symmetric(horizontal: 20, vertical: 14),
+        decoration: BoxDecoration(
+          gradient: enabled
+              ? LinearGradient(
+                  colors: [
+                    theme.okColor.withValues(alpha: 0.65),
+                    theme.okColor.withValues(alpha: 0.35),
+                  ],
+                  begin: Alignment.topLeft,
+                  end: Alignment.bottomRight,
+                )
+              : LinearGradient(
+                  colors: [theme.panelBorder, theme.panelBorder],
+                  begin: Alignment.topLeft,
+                  end: Alignment.bottomRight,
+                ),
+          borderRadius: BorderRadius.circular(8),
+          border: Border.all(
+            color: enabled ? theme.okColor : theme.panelBorder,
+            width: 1.5,
+          ),
+          boxShadow: enabled
+              ? [
+                  BoxShadow(
+                    color: theme.okColor
+                        .withValues(alpha: _pressed ? 0.2 : 0.45),
+                    blurRadius: _pressed ? 4 : 14,
+                    spreadRadius: _pressed ? 0 : 2,
+                  ),
+                ]
+              : [],
+        ),
+        child: Text(
+          'ENGAGE',
+          textAlign: TextAlign.center,
+          style: TextStyle(
+            color: enabled ? Colors.white : theme.textDisabled,
+            fontSize: 13,
+            fontWeight: FontWeight.w800,
+            letterSpacing: 1.2,
+            shadows: enabled
+                ? [Shadow(color: theme.okColor, blurRadius: 4)]
+                : null,
+          ),
+        ),
+      ),
+    );
+  }
+}