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/timer.tsx

@@ -0,0 +1,41 @@
+import { useRouter } from 'expo-router'
+import { useTimerEngine } from '@/src/features/timer'
+import { TimerDisplay } from '@/src/features/timer/components/TimerDisplay'
+
+export default function TimerScreen() {
+  const router = useRouter()
+  const timer = useTimerEngine()
+
+  function handleStart() {
+    timer.start()
+  }
+
+  function handleStop() {
+    timer.stop()
+    router.back()
+  }
+
+  return (
+    <TimerDisplay
+      state={{
+        phase: timer.phase,
+        secondsLeft: timer.secondsLeft,
+        currentRound: timer.currentRound,
+        totalRounds: timer.totalRounds,
+        currentCycle: timer.currentCycle,
+        totalCycles: timer.totalCycles,
+        isRunning: timer.isRunning,
+        isPaused: timer.isPaused,
+        totalElapsedSeconds: timer.totalElapsedSeconds,
+      }}
+      config={timer.config}
+      exerciseName="Burpees"
+      nextExerciseName="Squats"
+      onStart={handleStart}
+      onPause={timer.pause}
+      onResume={timer.resume}
+      onStop={handleStop}
+      onSkip={timer.skip}
+    />
+  )
+}