M0' scaffold: Godot 4.7 + GDExtension + libgr2 wired up (macOS)

- extension/: godot-cpp submodule (master, API 4.7), Metin2Model node
  registered, links xrender-poc/libgr2 via sibling-dir reference.
- project/: Godot 4.7 project — orbit camera + DirectionalLight3D (shadows)
  + WorldEnvironment/procedural sky + placeholder cube; main.gd probes the
  extension and can run libgr2 on a real .gr2 (MTGODOT_PROBE_GR2).
- build.sh one-shot build into project/bin/.
- docs/GODOT-POC-PLAN.md (phased: Phase 1 macOS -> M0'/M1/M2/M2.5).

Verified on this machine:
  [mtgodot] Metin2Model registered OK — libgr2 linked
  probe_gr2(warrior_cheongrin.gr2) -> gr2 OK: format_version=6 sections=8
  Metal Forward+ renders the scene (test/golden/m0-scaffold.png).

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

project/project.godot

@@ -0,0 +1,19 @@
+; Godot 4.7 project — mtgodot-poc Phase 1 (macOS)
+; See ../docs/GODOT-POC-PLAN.md
+
+config_version=5
+
+[application]
+
+config/name="mtgodot-poc"
+run/main_scene="res://main.tscn"
+config/features=PackedStringArray("4.7", "Forward Plus")
+
+[debug]
+
+gdscript/warnings/untyped_declaration=1
+
+[rendering]
+
+renderer/rendering_method="forward_plus"
+anti_aliasing/quality/msaa_3d=2