feat: 3D blobber dungeon generator (PR 1)

Replaces the 2D-only demo pipeline with a 3D cell-based blobber
generator. Per-cell face walls, per-material mesh emission, and a
GDExtension binding that returns a Dictionary with ArrayMesh surfaces
the demo consumes directly.

- src/blobber/: cell3d_t data model, dungeon container, pipeline that
  wraps the 2D generator per level and materializes into cell3d
- src/mesh/: face-quad emitter with per-material groups + .obj dump
- src/genesis3d_main.c: new CLI driving the blobber + mesh
- godot/: BrogueGen.generate_dungeon(seed, num_levels, depth) binding
  with dungeon_to_dict packing cells + mesh surfaces
- demo/: demo_blobber.tscn + dungeon_builder.gd, func_godot addon for
  the .map export path, point/entity templates, TrenchBroom docs
- Retired: old arcade/FPS demo scenes and their scripts, unused meshlib

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

demo/project.godot

@@ -15,8 +15,13 @@ compatibility/default_parent_skeleton_in_mesh_instance_3d=true
 [application]
 
 config/name="Brogue Genesis Demo"
+run/main_scene="uid://c8blbrbrbr001"
 config/features=PackedStringArray("4.6", "GL Compatibility")
 
+[editor_plugins]
+
+enabled=PackedStringArray("res://addons/func_godot/plugin.cfg")
+
 [rendering]
 
 renderer/rendering_method="gl_compatibility"