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/worldState.js

@@ -40,10 +40,6 @@ export const worldState = {
     solvedPuzzles: [],  // array of puzzleIds that have been solved
     activePuzzle: null,  // { puzzleId, requiredOutputs, doorX, doorY } or null when no puzzle active
 
-    // Return points — remembers where the player entered each map from
-    // Stack of { fromMap, fromX, fromY } — push on enter, pop on exit
-    returnPoints: [],
-
     // Game flags
     flags: {
         // Examples: