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>
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