Extract assets download and bundling to mk/assets/doom.mk

[pbalduino]

Parent Issue

Part of #352 - Makefile refactoring

Objective

Isolate optional assets download (currently Doom-related) and future asset bundling in a dedicated file.

Scope

Extract from root Makefile:

• Lines ~959: Optional assets download
• Asset bundling logic
• Asset-related targets
• Future asset management rules

Target Location

mk/assets/doom.mk

Implementation Steps

1. Create mk/assets/ directory structure
2. Extract Doom assets download rules
3. Move asset bundling logic
4. Update root Makefile to include mk/assets/doom.mk
5. Test asset download targets
6. Verify assets are correctly bundled

Dependencies

None - assets are optional and independent

Acceptance Criteria

• [ ] All asset-related rules in mk/assets/doom.mk
• [ ] Asset download targets work
• [ ] Assets correctly bundled into image
• [ ] Easy to add new asset types in the future
• [ ] No behavioral changes