Fix cull mode + isolate head-collapse to a libgr2 curve bug

Rendering fixes (real):
- m2_material shaders: cull_disabled -> cull_back (inside-of-mesh faces were
  z-winning and darkening/hiding the head under the ShaderMaterial path).
- drop `* COLOR` from the shader (meshes carry no ARRAY_COLOR).

Animation: convert gr2 global poses to Godot *local* bone poses and set via
set_bone_pose (was set_bone_global_pose). MTGODOT_VERIFY=1 confirms
get_bone_global_pose(i) == conv(world[i]) for all 75 bones (<=2.6e-5).

Head/upper-armor collapse on dance_1 is NOT a Godot-route bug:
- reproduced identically in xrender-poc's bgfx demo at the same anim/t
- reproduced by the new MTGODOT_CPUSKIN=1 reference path (same LBS math as
  xrender's validated skin_mesh)
- `general/wait` / `run` render correctly in all three
=> upstream libgr2 Curve::eval (degree-2 quaternion B-spline) vs Granny.
Demo default animation switched to general/wait; documented in MIDREVIEW §4
and README. Debug scaffolding gated behind MTGODOT_VERIFY / MTGODOT_CPUSKIN.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WaHYEY9rwLWt21PULiYjeJ

extension/src/metin2_model.h

@@ -1,5 +1,6 @@
 #pragma once
 
+#include <godot_cpp/classes/array_mesh.hpp>
 #include <godot_cpp/classes/image_texture.hpp>
 #include <godot_cpp/classes/node3d.hpp>
 #include <godot_cpp/classes/ref.hpp>
@@ -16,7 +17,6 @@
 namespace godot {
 class Skeleton3D;
 class MeshInstance3D;
-class ImageTexture;
 } // namespace godot
 
 namespace mtgodot {
@@ -70,6 +70,14 @@ public:
 	// --- C++ accessors for Metin2AnimPlayer (same extension) ---
 	const gr2::Skeleton *gr2_skeleton() const;
 	godot::Skeleton3D *skeleton_node() const { return skel; }
+	const gr2::FileInfo *gr2_fileinfo() const;
+
+	// CPU linear-blend skinning: rewrite ArrayMesh vertex regions from the
+	// per-bone deformer matrices (gr2::sample_pose's `skin` output). Debug/
+	// reference path; bypasses Godot's Skeleton3D skinning entirely.
+	void cpu_skin(const std::vector<gr2::Mat4> &skin);
+	bool has_cpu_skin_mesh() const { return cpu_mesh.is_valid(); }
+	void enable_cpu_skin(bool on);
 
 	// libgr2 sanity probe (kept from M0').
 	godot::String probe_gr2(const godot::String &path) const;
@@ -95,6 +103,9 @@ private:
 	godot::String last_info;
 	godot::HashMap<godot::String, godot::Ref<godot::ImageTexture>> tex_cache;
 
+	godot::Ref<godot::ArrayMesh> cpu_mesh;   // rebuilt each frame in cpu_skin mode
+	std::vector<int> cpu_surf_mesh;          // surface -> gr2 mesh index
+
 	void _clear_children();
 	void _apply_materials();
 	godot::String _guess_texture_dir() const;