Step 252: diagnostic delta streaming (getDiagnosticsDelta, version tracking)

DiagnosticVersionTracker records snapshots and computes added/removed deltas
between versions. Agents can efficiently check if their fix resolved an error
without re-fetching all diagnostics. 21 MCP tools total.

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

editor/src/HeadlessEditorState.h

@@ -67,6 +67,7 @@ struct HeadlessBufferState {
     Orchestrator         orchestrator;
     IncrementalOptimizer incrementalOptimizer;
     ASTVersionTracker    versionTracker;
+    DiagnosticVersionTracker diagTracker;
     std::string          language    = "python";
     std::string          path        = "(untitled)";
     std::string          editBuf;