Protect library directory and shared buffer access

Move the library directory and shared vertex/index buffer handles into the
protected source and rendering groups. Expose one GetBuffers accessor returning
const references to both buffers, and use structured binding in BindMesh while
preserving GL binding and uncached-mesh fallback behavior.

Add GetLibraryFilePath for mapping-file lookups and update the LXF and BrickLink
callers. Keep file opening and bundled-resource fallback in those callers so the
library only resolves the path and exposes no mutable directory state.

Validation: warning-free macOS Qt 6.10 Debug build, all 45 offscreen lifecycle
checks and diff whitespace checks pass. No remaining external field accesses
for the protected directory or buffers. GPU rendering was not tested.

common/lc_bricklink.cpp

@@ -11,7 +11,7 @@ static QJsonDocument lcLoadBrickLinkMapping()
 {
 	std::map<std::string, std::string> Remapping;
 
-	QFile File(lcGetPiecesLibrary()->mLibraryDir.absoluteFilePath(QLatin1String("ldraw2bl.txt")));
+	QFile File(lcGetPiecesLibrary()->GetLibraryFilePath(QLatin1String("ldraw2bl.txt")));
 	QByteArray Data;
 
 	if (File.open(QIODevice::ReadOnly))

common/lc_context.cpp

@@ -1519,12 +1519,11 @@ void lcContext::SetIndexBufferPointer(const void* IndexBuffer)
 
 void lcContext::BindMesh(const lcMesh* Mesh)
 {
-	const lcPiecesLibrary* const Library = lcGetPiecesLibrary();
-
 	if (Mesh->mVertexCacheOffset != -1)
 	{
-		const GLuint VertexBufferObject = Library->mVertexBuffer.Object;
-		const GLuint IndexBufferObject = Library->mIndexBuffer.Object;
+		const auto [VertexBuffer, IndexBuffer] = lcGetPiecesLibrary()->GetBuffers();
+		const GLuint VertexBufferObject = VertexBuffer.Object;
+		const GLuint IndexBufferObject = IndexBuffer.Object;
 
 		if (VertexBufferObject != mVertexBufferObject)
 		{

common/lc_library.h

@@ -145,6 +145,12 @@ public:
 	// Loads the bundled fallback library and its default settings.
 	bool LoadBuiltinPieceLibrary();
 
+	// Resolves a file path relative to the library directory without opening it.
+	QString GetLibraryFilePath(const QString& FileName) const
+	{
+		return mLibraryDir.absoluteFilePath(FileName);
+	}
+
 	// Returns the official archive count for update checks, or zero for custom/directory libraries.
 	int GetOfficialPieceCount() const
 	{
@@ -292,17 +298,18 @@ public:
 	// Rebuilds shared mesh buffers when dirty, using the supplied render context.
 	void UpdateBuffers(lcContext* Context);
 
+	// Returns borrowed read-only references to the shared vertex and index buffers.
+	std::pair<const lcVertexBuffer&, const lcIndexBuffer&> GetBuffers() const
+	{
+		return { mVertexBuffer, mIndexBuffer };
+	}
+
 	// Checks unused library parts under the existing zero-reference release policy.
 	void UnloadUnusedParts();
 
 	// Returns the borrowed placeholder mesh for pending or failed piece geometry.
 	lcMesh* GetLoadingMesh() const;
 
-	QDir mLibraryDir;
-
-	lcVertexBuffer mVertexBuffer;
-	lcIndexBuffer mIndexBuffer;
-
 signals:
 	void AssetStatusChanged();
 	void PartLoaded(lcPieceData* PieceData);
@@ -396,7 +403,8 @@ protected:
 
 	void ReleaseBuffers();
 
-	// Archive readers and discovered primitive sources; workers drain before replacement or teardown.
+	// Library directory, archive readers and primitive sources; workers drain before replacement or teardown.
+	QDir mLibraryDir;
 	std::unique_ptr<lcZipFile> mZipFiles[static_cast<int>(lcZipFileType::Count)];
 	std::vector<std::unique_ptr<lcLibrarySource>> mSources;
 	bool mHasUnofficial;
@@ -432,7 +440,9 @@ protected:
 	QString mCachePath;
 	qint64 mArchiveCheckSum[4];
 
-	// Placeholder geometry and deferred shared-buffer repacking state.
+	// Shared GPU buffers, placeholder geometry and deferred repacking state.
+	lcVertexBuffer mVertexBuffer;
+	lcIndexBuffer mIndexBuffer;
 	bool mBuffersDirty;
 	std::unique_ptr<lcMesh> mLoadingMesh;
 	bool mStreamingBuffersPending = false;

common/lc_lxf.cpp

@@ -9,7 +9,7 @@
 
 static bool lcLoadLDrawXML(std::map<int, int>& MaterialTable, std::map<int, std::string>& BrickTable, std::map<std::string, std::pair<lcVector3, lcVector4>>& TransformTable)
 {
-	QFile File(lcGetPiecesLibrary()->mLibraryDir.absoluteFilePath(QLatin1String("ldraw.xml")));
+	QFile File(lcGetPiecesLibrary()->GetLibraryFilePath(QLatin1String("ldraw.xml")));
 	QByteArray Data;
 
 	if (File.open(QIODevice::ReadOnly))