feat: bidirectional door system + editor bi-link tool

Replace spawn-based map transitions with explicit bidirectional door
links. Every exit now requires targetX/targetY — spawn is only used
for initial game start. Remove returnPoints stack from worldState.

Editor improvements:
- New "Bi-Link" tool creates paired exits on both maps at once
- Exit list shows target coordinates and warns if missing
- Canvas renders target info labels below exit tiles
- Properties panel handles game ID ↔ editor ID mapping

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

js/world/maps.js

@@ -32,9 +32,9 @@ const labMap = {
     wallSet: buildWallSet(labWalls),
 
     exits: [
-        // No targetX/targetY — uses stored return point (the door the player entered from)
-        { x: 4, y: 11, targetMap: 'town' },
-        { x: 5, y: 11, targetMap: 'town' },
+        // Bidirectional: these doors return to the specific town door (12,12 = tile in front of lab entrance)
+        { x: 4, y: 11, targetMap: 'town', targetX: 12, targetY: 12 },
+        { x: 5, y: 11, targetMap: 'town', targetX: 12, targetY: 12 },
     ],
 
     npcs: [