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/scripts/dungeon_builder.gd

@@ -0,0 +1,92 @@
+extends Node3D
+
+# Blobber dungeon builder. Calls BrogueGen.generate_dungeon() and assembles
+# per-material MeshInstance3D children from the returned mesh surface arrays.
+# Material ids mirror src/mesh/material_ids.h.
+
+const MAT_STONE_FLOOR   := 0
+const MAT_STONE_CEILING := 1
+const MAT_STONE_WALL    := 2
+const MAT_DOOR_FLOOR    := 3
+const MAT_STAIR_UP      := 4
+const MAT_STAIR_DOWN    := 5
+const MAT_WATER         := 6
+const MAT_LAVA          := 7
+const MAT_BRIDGE        := 8
+const MAT_CAVE_FLOOR    := 9
+const MAT_CAVE_CEILING  := 10
+const MAT_CAVE_WALL     := 11
+
+@export var seed_value: int = 42
+@export var num_levels: int = 1
+@export var depth: int = 1
+@export var regenerate_on_ready: bool = true
+
+var _materials: Dictionary = {}
+
+func _ready() -> void:
+	_materials = _build_materials()
+	if regenerate_on_ready:
+		regenerate()
+
+func regenerate() -> void:
+	for child in get_children():
+		child.queue_free()
+
+	var gen := BrogueGen.new()
+	var dungeon: Dictionary = gen.generate_dungeon(seed_value, num_levels, depth)
+	gen.free()
+
+	if dungeon.is_empty():
+		push_error("generate_dungeon returned empty dictionary")
+		return
+
+	var meshes: Array = dungeon.get("meshes", [])
+	for entry_v in meshes:
+		var entry: Dictionary = entry_v
+		var material_id: int = entry.get("material", -1)
+		var arrays: Array = entry.get("arrays", [])
+		if material_id < 0 or arrays.is_empty():
+			continue
+
+		var mesh := ArrayMesh.new()
+		mesh.add_surface_from_arrays(Mesh.PRIMITIVE_TRIANGLES, arrays)
+		mesh.surface_set_material(0, _materials.get(material_id, _fallback_material()))
+
+		var mi := MeshInstance3D.new()
+		mi.mesh = mesh
+		mi.name = "Surface_%d" % material_id
+		add_child(mi)
+
+	var dims: Vector3i = dungeon.get("dimensions", Vector3i(79, 1, 29))
+	var levels: Array = dungeon.get("levels", [])
+	print("Dungeon built: seed=%d dims=%s surfaces=%d levels=%d" %
+		[seed_value, dims, meshes.size(), levels.size()])
+
+# Distinct albedo colors per material for instant visual debugging.
+# Later PRs replace these with proper StandardMaterial3D + textures.
+func _build_materials() -> Dictionary:
+	var m := {}
+	m[MAT_STONE_FLOOR]   = _flat(Color(0.45, 0.42, 0.38))
+	m[MAT_STONE_CEILING] = _flat(Color(0.30, 0.28, 0.26))
+	m[MAT_STONE_WALL]    = _flat(Color(0.55, 0.50, 0.45))
+	m[MAT_DOOR_FLOOR]    = _flat(Color(0.62, 0.38, 0.18))
+	m[MAT_STAIR_UP]      = _flat(Color(0.20, 0.75, 0.40))
+	m[MAT_STAIR_DOWN]    = _flat(Color(0.85, 0.25, 0.20))
+	m[MAT_WATER]         = _flat(Color(0.15, 0.40, 0.75))
+	m[MAT_LAVA]          = _flat(Color(0.95, 0.40, 0.10))
+	m[MAT_BRIDGE]        = _flat(Color(0.50, 0.35, 0.20))
+	m[MAT_CAVE_FLOOR]    = _flat(Color(0.35, 0.32, 0.28))
+	m[MAT_CAVE_CEILING]  = _flat(Color(0.22, 0.20, 0.18))
+	m[MAT_CAVE_WALL]     = _flat(Color(0.42, 0.38, 0.34))
+	return m
+
+func _flat(c: Color) -> StandardMaterial3D:
+	var sm := StandardMaterial3D.new()
+	sm.albedo_color = c
+	sm.roughness = 0.85
+	sm.metallic = 0.0
+	return sm
+
+func _fallback_material() -> StandardMaterial3D:
+	return _flat(Color.MAGENTA)