port 2V2-d.2: Granny runtime on libgr2, main character model animates

Implement the Granny SDK 2.11 API the client uses on top of libgr2
(platform/EterGrnLib/GrannyRuntime.cpp, shim granny.h declared from the
SDK header): file sections, type-definition vertex conversion, material
textures by Usage, mesh/bone bindings, deformer, control clock/loop/ease
and weighted SRT sampling. Port EterGrnLib (LODController, Material, Mesh,
Model, ModelInstance*, Motion, Thing, ThingInstance, Util) plus EterLib
GrpObjectInstance/GrpCollisionObject/CollisionData verbatim and drop their
platform skeletons and pending stand-ins; register the gr2 resource
factory; vertex/index buffers are CPU-side until the 2V2-e render adapter.

app.UpdateGame/RenderGame now run 40250's bodies, reaching the
CPythonApplication members through their singletons (the platform has no
app object); CPythonGraphic joins GameSingletons in member order, and
GetMousePosition reads the host cursor.

port.login_flow asserts the main instance's PART_MAIN model has meshes,
its diffuse image loaded and the WAIT motion moves the bones between
frames; offline and live pass, port_gate macos PASS.

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

extension/src/platform/EterLib/MSWindow.cpp

@@ -4,6 +4,7 @@
 #include "EterLib/MSWindow.h"
 
 #include "../PlatformStub.h"
+#include "HostCursor.h"
 
 CMSWindow::CMSWindow()
 {
@@ -78,9 +79,19 @@ auto CMSWindow::IsActive() -> bool
 	return mt_platform_stub_return<bool>();
 }
 
-auto CMSWindow::GetMousePosition(POINT *) -> void
+namespace {
+POINT g_host_cursor = {0, 0};
+}
+
+void MtHostSetCursor(int x, int y)
 {
-	MT_PLATFORM_STUB();
+	g_host_cursor.x = x;
+	g_host_cursor.y = y;
+}
+
+auto CMSWindow::GetMousePosition(POINT * ppt) -> void
+{
+	*ppt = g_host_cursor;
 }
 
 auto CMSWindow::GetClientRect(RECT *) -> void