refactor: remove hardcoded barriers from predefined maps

[nickmeinhold]

Problem or Motivation

Predefined maps define barriers as hardcoded Point<int> lists. When a room is edited and saved, these get merged with auto-barriers from painted tiles and the distinction is lost. On reload, the wall occlusion filter can't tell them apart, causing barriers to become invisible.

Proposed Solution

Clear the barriers lists from all predefined maps in predefined_maps.dart. Barriers should come exclusively from tiles painted in the editor (object-layer auto-barriers or structure-tool painting).

The L-Room and other maps with predefined barriers will need their walls re-created using the tile editor.

Acceptance Criteria

• All predefined maps have empty barriers lists
• Barrier rendering still works for editor-painted barriers
• Tests updated for new barrier counts
• All tests pass