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/worldspawn.tres

@@ -0,0 +1,18 @@
+[gd_resource type="Resource" script_class="FuncGodotFGDSolidClass" load_steps=4 format=3 uid="uid://bdji3873bg32h"]
+
+[ext_resource type="Script" uid="uid://5cow84q03m6a" path="res://addons/func_godot/src/fgd/func_godot_fgd_solid_class.gd" id="1_62t8m"]
+[ext_resource type="Resource" uid="uid://doo4ly322b4jc" path="res://addons/func_godot/fgd/vertex_merge_distance_base.tres" id="1_h1046"]
+[ext_resource type="Resource" uid="uid://bcdsueg5pysfq" path="res://addons/func_godot/fgd/cull_interior_faces.tres" id="2_ky6lr"]
+
+[resource]
+script = ExtResource("1_62t8m")
+spawn_type = 0
+origin_type = 1
+collision_mask = 0
+classname = "worldspawn"
+description = "Default static world geometry. Builds a StaticBody3D with a single MeshInstance3D and a single convex CollisionShape3D shape."
+base_classes = Array[Resource]([ExtResource("1_h1046"), ExtResource("2_ky6lr")])
+meta_properties = Dictionary[String, Variant]({
+"color": Color(0.8, 0.8, 0.8, 1)
+})
+node_class = "StaticBody3D"