Step 258: project model and workspace indexing (Phase 9d start)

Adds multi-buffer management via openFile/closeFile/listBuffers/setActiveBuffer
RPC methods, plus indexWorkspace for scanning workspace files without opening
them. Language auto-detected from extension. 27 MCP tools total (was 22).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

editor/src/HeadlessEditorState.h

@@ -33,6 +33,7 @@
 #include "EditorModePolicy.h"
 #include "PrimitivesRegistry.h"
 #include "SemanticTags.h"
+#include "ProjectState.h"
 
 #include <nlohmann/json.hpp>
 #include <string>
@@ -87,6 +88,7 @@ struct HeadlessEditorState {
     HeadlessBufferState* activeBuffer = nullptr;
     HeadlessAgentState   agent;
     HeadlessLibraryState library;
+    ProjectState         project;
     std::string          workspaceRoot;
     std::string          defaultLanguage = "python";
     bool                 verbose = false;