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

CMakeLists.txt

@@ -0,0 +1,21 @@
+# mtgodot-poc — top level
+# Phase 1 (macOS) scaffolding. See docs/GODOT-POC-PLAN.md §M0'.
+cmake_minimum_required(VERSION 3.20)
+project(mtgodot_poc CXX C)
+
+set(CMAKE_CXX_STANDARD 20)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+set(CMAKE_C_STANDARD 17)
+
+if(NOT CMAKE_BUILD_TYPE AND NOT CMAKE_CONFIGURATION_TYPES)
+    set(CMAKE_BUILD_TYPE Debug CACHE STRING "" FORCE)
+endif()
+
+set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
+
+# Phase 1 is macOS-only; fail early anywhere else so nobody wastes time.
+if(NOT APPLE)
+    message(FATAL_ERROR "mtgodot-poc Phase 1 targets macOS only (see docs/GODOT-POC-PLAN.md §00).")
+endif()
+
+add_subdirectory(extension)