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/textures/default_material.tres

@@ -0,0 +1,8 @@
+[gd_resource type="StandardMaterial3D" load_steps=2 format=3 uid="uid://cvex6toty8yn7"]
+
+[ext_resource type="Texture2D" uid="uid://cyg2snr1w5xw5" path="res://addons/func_godot/textures/default_texture.png" id="1_ncj77"]
+
+[resource]
+albedo_texture = ExtResource("1_ncj77")
+metallic_specular = 0.0
+texture_filter = 2