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/addons/func_godot/fgd/cull_interior_faces.tres

@@ -0,0 +1,15 @@
+[gd_resource type="Resource" script_class="FuncGodotFGDBaseClass" load_steps=2 format=3 uid="uid://bcdsueg5pysfq"]
+
+[ext_resource type="Script" uid="uid://ck575aqs1sbrb" path="res://addons/func_godot/src/fgd/func_godot_fgd_base_class.gd" id="1_21jph"]
+
+[resource]
+script = ExtResource("1_21jph")
+classname = "CullInteriorFaces"
+description = "Cull interior faces option for SolidClass Geometry"
+class_properties = Dictionary[String, Variant]({
+"_cull_interior_faces": false
+})
+class_property_descriptions = Dictionary[String, Variant]({
+"_cull_interior_faces": "If true, cull interior faces with matching vertices or faces that are flush within a larger face. Note: This has a performance impact that scales with how many brushes are in the brush entity."
+})
+metadata/_custom_type_script = "uid://cgkrrgcimlr8y"