feat: add F3 debug overlay for collision visualization

Press F3 to toggle a debug overlay that shows:
- Red tiles: walls (collision)
- Green tiles: exits (map transitions)
- Yellow tiles: interactions (workshop, signs, doors)
- Purple tiles: NPCs
- Green border: current player tile
- Coordinate labels on nearby tiles
- Legend bar with player position and current map

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

js/world/worldInput.js

@@ -1,6 +1,7 @@
 // worldInput.js - Keyboard input for world mode
 import { worldState, advanceDialog, startDialog } from './worldState.js';
 import { getMap, getInteraction, getNPC, getExit, isWalkable } from './maps.js';
+import { toggleDebug } from './worldRenderer.js';
 
 const keysDown = new Set();
 let interactionHandler = null;
@@ -35,6 +36,13 @@ function onKeyDown(e) {
         return;
     }
 
+    // Debug overlay toggle (F3)
+    if (key === 'F3') {
+        e.preventDefault();
+        toggleDebug();
+        return;
+    }
+
     // Workshop shortcut (TAB)
     if (key === 'Tab') {
         e.preventDefault();