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

README.md

@@ -26,8 +26,11 @@
   （`test/golden/m1-bindpose.png`）。多骨架：`warrior_lord`(v6,74)、`assassin`(v6,95,17 表面)、
   `shaman_lord`(**v7**,93) 均正确加载渲染。
 - [x] **M2** — 骨骼动画。`Metin2AnimPlayer` `_process` 里 `gr2::sample_pose`（跨文件）
-  → `Skeleton3D.set_bone_global_pose`，Godot GPU 蒙皮。`dance01`（28.3s / 74 track）
-  播放，`selfcheck` 25 采样 0 NaN，t=2/9/16/23 姿势各异（`test/golden/m2c-*.png`）。
+  → 逐骨 `Skeleton3D.set_bone_pose`（局部），Godot GPU 蒙皮。`general/wait` 头 / 护甲
+  对齐正确（`test/golden/m2-wait.png`）；`get_bone_global_pose` vs `conv(world)` 全骨 ≤2.6e-5；
+  CPU-LBS 参考路径（`MTGODOT_CPUSKIN=1`）与 GPU 蒙皮一致；`selfcheck` 0 NaN。
+  ⚠️ `dance_1` 等表情动作头 / 颈塌陷 —— **libgr2 曲线解码 bug，xrender-poc bgfx demo 同样复现**
+  （见 `docs/MIDREVIEW.md` §4）。
 - [~] **M2.5** — 材质与观感保真。`ShaderMaterial`（modulate tex×COLOR×light、
   opaque/alpha/alpha-test/add 四模式）+ 目录扫描贴图解析 + 方向光阴影 + procedural sky
   + 引擎雾（`test/golden/m25-*.png`）。**门禁未闭**：多材质槽 → 贴图/混合的精确映射需