feat: return-to-door system for map exits

When entering an interior (e.g. lab from town), the game saves the
player's current position as a return point. When exiting, if the
exit has no explicit targetX/targetY, the system pops the stored
return point and warps back to that exact position.

This means interior exits just need targetMap — the player always
returns to the specific door they entered from, not a hardcoded
position. Falls back to the destination map's spawn if no return
point is stored.

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

js/world/gameMode.js

@@ -113,8 +113,38 @@ function handleInteraction(event) {
 
         case 'mapExit': {
             const { targetMap, targetX, targetY } = event.data;
-            warpToMap(targetMap, targetX, targetY);
-            console.log(`[gameMode] warped to ${targetMap} (${targetX}, ${targetY})`);
+            const p = worldState.player;
+
+            // Save return point: where the player is NOW (one tile back from the exit)
+            // so when they leave the target map, they return here
+            worldState.returnPoints.push({
+                fromMap: worldState.currentMap,
+                fromX: p.x,
+                fromY: p.y
+            });
+
+            // If exit has explicit coordinates, use them
+            // Otherwise, check for a stored return point for the target map
+            if (targetX != null && targetY != null) {
+                warpToMap(targetMap, targetX, targetY);
+            } else {
+                // Pop the most recent return point for this map
+                const retIdx = worldState.returnPoints.findLastIndex(
+                    rp => rp.fromMap === targetMap
+                );
+                if (retIdx >= 0) {
+                    const ret = worldState.returnPoints[retIdx];
+                    worldState.returnPoints.splice(retIdx, 1);
+                    warpToMap(ret.fromMap, ret.fromX, ret.fromY);
+                } else {
+                    // Fallback: use map spawn point
+                    const destMap = getMap(targetMap);
+                    const sx = destMap?.spawn?.x ?? 0;
+                    const sy = destMap?.spawn?.y ?? 0;
+                    warpToMap(targetMap, sx, sy);
+                }
+            }
+            console.log(`[gameMode] warped to ${worldState.currentMap} (${worldState.player.x}, ${worldState.player.y})`);
             break;
         }