Step 254: response budget system (truncation, continuation, pagination)

Optional budget parameter on getAST and getDiagnostics. Responses exceeding
the budget are truncated with continuation tokens for paginated follow-up.
Binary search finds max array elements fitting the budget. Diagnostics sorted
by severity (errors first) before truncation.

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

editor/src/HeadlessEditorState.h

@@ -25,6 +25,7 @@
 #include "FileOperations.h"
 #include "CompactAST.h"
 #include "StructuredDiagnostics.h"
+#include "ResponseBudget.h"
 #include "Orchestrator.h"
 #include "IncrementalOptimizer.h"
 #include "TextASTSync.h"