feat: timer engine + full-screen timer UI

Implement the core timer feature following the src/features/ architecture:

- useTimerEngine hook: drift-free Date.now() delta countdown (100ms tick),
  explicit state machine (IDLE → GET_READY → WORK → REST → COMPLETE),
  event emitter for external consumers (PHASE_CHANGED, ROUND_COMPLETED,
  COUNTDOWN_TICK, SESSION_COMPLETE), auto-pause on AppState interruption
  (phone calls, background), expo-keep-awake during session
- TimerDisplay component: full-screen animated UI with 600ms color
  transitions between phases, pulse animation on countdown, flash red
  on last 3 seconds, round progress dots, IDLE/active/COMPLETE views
- TimerControls component: stop/pause-resume/skip buttons with Ionicons
- Timer route (app/timer.tsx): fullScreenModal wiring hook → display
- Home screen: dark theme with START button navigating to /timer
- Project docs: CLAUDE.md (constitution), PRD v1.1, skill files
- Shared constants: PHASE_COLORS, TIMER_DEFAULTS, formatTime utility
- Types: TimerPhase, TimerState, TimerConfig, TimerActions, TimerEvent

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

package.json

@@ -20,6 +20,7 @@
     "expo-font": "~14.0.11",
     "expo-haptics": "~15.0.8",
     "expo-image": "~3.0.11",
+    "expo-keep-awake": "~15.0.8",
     "expo-linking": "~8.0.11",
     "expo-router": "~6.0.23",
     "expo-splash-screen": "~31.0.13",
@@ -31,11 +32,11 @@
     "react-dom": "19.1.0",
     "react-native": "0.81.5",
     "react-native-gesture-handler": "~2.28.0",
-    "react-native-worklets": "0.5.1",
     "react-native-reanimated": "~4.1.1",
     "react-native-safe-area-context": "~5.6.0",
     "react-native-screens": "~4.16.0",
-    "react-native-web": "~0.21.0"
+    "react-native-web": "~0.21.0",
+    "react-native-worklets": "0.5.1"
   },
   "devDependencies": {
     "@types/react": "~19.1.0",