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>

app/_layout.tsx

@@ -16,6 +16,14 @@ export default function RootLayout() {
     <ThemeProvider value={colorScheme === 'dark' ? DarkTheme : DefaultTheme}>
       <Stack>
         <Stack.Screen name="(tabs)" options={{ headerShown: false }} />
+        <Stack.Screen
+          name="timer"
+          options={{
+            headerShown: false,
+            presentation: 'fullScreenModal',
+            animation: 'fade',
+          }}
+        />
         <Stack.Screen name="modal" options={{ presentation: 'modal', title: 'Modal' }} />
       </Stack>
       <StatusBar style="auto" />