M2.5: wire libgr2 dump_materials (opt-in), keep filename heuristic default

- Metin2Model stores gr2::dump_materials() and can resolve surface textures
  via tri_groups[0].material_index -> material.diffuse_texture basename,
  gated behind use_gr2_materials (default off): the mapping is unverified and
  currently regresses warrior (body surfaces pick the face texture), so the
  filename-convention + dir-scan path stays the default.
- Property + setter for use_gr2_materials so it can be A/B'd once a Granny
  oracle exists. Logged: "gr2 materials: N (M with diffuse name)".

warrior/assassin/shaman render unchanged from the heuristic path.
docs/GODOT-POC-PLAN.md §M2.5 T2.5.6 gap list already covers the open item.

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

extension/src/metin2_model.h

@@ -11,6 +11,7 @@
 
 #include <memory>
 #include <optional>
+#include <vector>
 
 namespace godot {
 class Skeleton3D;
@@ -54,6 +55,9 @@ public:
 	void set_material_mode(const godot::String &m);
 	godot::String get_material_mode() const { return material_mode; }
 
+	void set_use_gr2_materials(bool v);
+	bool get_use_gr2_materials() const { return use_gr2_materials; }
+
 	// Explicit per-surface texture path override (index -> absolute .dds path).
 	void set_surface_texture(int surface, const godot::String &path);
 
@@ -81,9 +85,11 @@ private:
 	bool flip_winding = false;
 
 	godot::String material_mode = "metin2"; // "metin2" (ShaderMaterial) | "standard"
+	bool use_gr2_materials = false; // drive textures from gr2 material names (unverified)
 	godot::PackedStringArray surface_tex_override;
 
 	std::shared_ptr<std::optional<gr2::File>> file; // shared_ptr so accessor stays valid
+	std::vector<gr2::MaterialInfo> materials;
 	godot::Skeleton3D *skel = nullptr;
 	godot::MeshInstance3D *mi = nullptr;
 	godot::String last_info;