SpeedTreeLib: port 40250 wrapper/forest verbatim over a CSpeedTreeRT stand-in

Delete the platform proxy tree renderer and copy SpeedTreeWrapper, SpeedTreeForest,
SpeedTreeForestDirectX8, SpeedTreeConfig.h and VertexShaders.h from 40250. The SDK
SpeedTreeRT.h public API becomes the shim; the closed CSpeedTreeRT is reimplemented in
platform/SpeedTreeLib/SpeedTreeRT.cpp (procedural geometry, static lighting, single LOD,
DIVERGENT). Shims gain the D3D8 vertex shader declaration tokens, D3D_OK,
CreateVertexShader/DeleteVertexShader, ID3DXBuffer/D3DXAssembleShader, MessageBox and
__min/__max.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

extension/src/port/common/Win32Crt.h

@@ -72,6 +72,21 @@ BOOL FindNextFile(HANDLE handle, WIN32_FIND_DATA* data);
 BOOL FindClose(HANDLE handle);
 void Sleep(DWORD milliseconds);
 
+// MSVC <stdlib.h> __min / __max
+#ifndef __min
+#define __min(a, b) (((a) < (b)) ? (a) : (b))
+#endif
+#ifndef __max
+#define __max(a, b) (((a) > (b)) ? (a) : (b))
+#endif
+
+// user32 MessageBox (ANSI): no dialog on these hosts; writes caption and text to stderr, returns IDOK.
+#define MB_OK 0x00000000L
+#define MB_ICONHAND 0x00000010L
+#define MB_ICONSTOP MB_ICONHAND
+#define IDOK 1
+int MessageBox(HWND hWnd, LPCSTR lpText, LPCSTR lpCaption, UINT uType);
+
 // winmm `timeGetTime`: milliseconds since system start, wrapping at 2^32.
 DWORD timeGetTime();
 DWORD GetTickCount();