fix: make spawn optional — only required for initial map

Spawn can now be deleted in the editor (click same tile with Spawn tool,
use Delete tool, or press Delete key). Interior maps no longer have
spawn objects. The editor shows "None" when no spawn is set, and the
generated maps.js omits the spawn field for maps without one.

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

js/world/maps.js

@@ -28,7 +28,7 @@ const labMap = {
     image: 'map:lab',
     widthTiles: 10,
     heightTiles: 12,
-    spawn: { x: 4, y: 10 },
+    // No spawn — player enters via door from town
     wallSet: buildWallSet(labWalls),
 
     exits: [
@@ -110,7 +110,6 @@ const houseA1fMap = {
     image: 'map:house-a-1f',
     widthTiles: 8,
     heightTiles: 8,
-    spawn: { x: 0, y: 0 },
     wallSet: buildWallSet(houseA1fWalls),
 
     exits: [
@@ -134,7 +133,6 @@ const route1Map = {
     image: 'map:route-1',
     widthTiles: 20,
     heightTiles: 36,
-    spawn: { x: 0, y: 0 },
     wallSet: buildWallSet(route1Walls),
 
     exits: [