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/func_godot_default_map_settings.tres

@@ -0,0 +1,9 @@
+[gd_resource type="Resource" script_class="FuncGodotMapSettings" load_steps=5 format=3 uid="uid://bkhxcqsquw1yg"]
+
+[ext_resource type="Material" uid="uid://cvex6toty8yn7" path="res://addons/func_godot/textures/default_material.tres" id="1_8l5wm"]
+[ext_resource type="Script" uid="uid://38q6k0ctahjn" path="res://addons/func_godot/src/map/func_godot_map_settings.gd" id="1_dlf23"]
+[ext_resource type="Resource" uid="uid://crgpdahjaj" path="res://addons/func_godot/fgd/func_godot_fgd.tres" id="2_hf4oi"]
+[ext_resource type="Script" uid="uid://cij36hpqc46c" path="res://addons/func_godot/src/import/quake_wad_file.gd" id="4_576s4"]
+
+[resource]
+script = ExtResource("1_dlf23")