fix(complement-race): track previous position to detect route threshold crossing

PROBLEM:
Previous fix broke route progression - train would never advance to next route.

ROOT CAUSE:
After removing the dual game loop, I changed line 97 to:
  const trainPosition = state.trainPosition

This meant the route completion check (lines 296-299) became:
  if (
    state.trainPosition >= threshold &&
    state.trainPosition < threshold  // Same variable!
  )

This is ALWAYS FALSE because a value can't be both >= and < threshold.

The previous code worked because:
- trainPosition was the newly calculated position for this frame
- state.trainPosition was the previous frame's position
- So it could detect threshold crossing: newPos >= threshold && oldPos < threshold

FIX:
1. Added previousTrainPositionRef to track position from previous frame (line 48)
2. Updated route completion check to use previousPosition instead of state.trainPosition (line 300)
3. Store current position in ref at end of each frame (line 323)
4. Reset previousPosition when route changes (line 82)
5. Added debug logging when route completes (line 310-312)

Now the check correctly detects:
  if (
    trainPosition >= threshold &&      // Current position
    previousPosition < threshold       // Previous position
  )

This properly detects when the train crosses the exit threshold.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

apps/web/src/app/arcade/complement-race/hooks/useSteamJourney.ts

@@ -45,6 +45,7 @@ export function useSteamJourney() {
   const routeExitThresholdRef = useRef<number>(107) // Default for 1 car: 100 + 7
   const missedPassengersRef = useRef<Set<string>>(new Set()) // Track which passengers have been logged as missed
   const pendingBoardingRef = useRef<Set<string>>(new Set()) // Track passengers with pending boarding requests across frames
+  const previousTrainPositionRef = useRef<number>(0) // Track previous position to detect threshold crossings
 
   // Initialize game start time
   useEffect(() => {
@@ -78,6 +79,7 @@ export function useSteamJourney() {
   useEffect(() => {
     pendingBoardingRef.current.clear()
     missedPassengersRef.current.clear()
+    previousTrainPositionRef.current = 0 // Reset previous position for new route
   }, [state.currentRoute])
 
   // Momentum decay and position update loop
@@ -292,10 +294,11 @@ export function useSteamJourney() {
 
       // Check for route completion (entire train exits tunnel)
       const ENTIRE_TRAIN_EXIT_THRESHOLD = routeExitThresholdRef.current
+      const previousPosition = previousTrainPositionRef.current
 
       if (
         trainPosition >= ENTIRE_TRAIN_EXIT_THRESHOLD &&
-        state.trainPosition < ENTIRE_TRAIN_EXIT_THRESHOLD
+        previousPosition < ENTIRE_TRAIN_EXIT_THRESHOLD
       ) {
         // Play celebration whistle
         playSound('train_whistle', 0.6)
@@ -305,6 +308,9 @@ export function useSteamJourney() {
 
         // Auto-advance to next route
         const nextRoute = state.currentRoute + 1
+        console.log(
+          `🏁 ROUTE COMPLETE: Train crossed exit threshold (${trainPosition.toFixed(1)} >= ${ENTIRE_TRAIN_EXIT_THRESHOLD}). Advancing to Route ${nextRoute}`
+        )
         dispatch({
           type: 'START_NEW_ROUTE',
           routeNumber: nextRoute,
@@ -313,6 +319,9 @@ export function useSteamJourney() {
 
         // Note: New passengers will be generated by the server when it handles START_NEW_ROUTE
       }
+
+      // Update previous position for next frame
+      previousTrainPositionRef.current = trainPosition
     }, UPDATE_INTERVAL)
 
     return () => clearInterval(interval)