feat(editor): compile-diagnostics seam — publish raw build output for editor markers

frontend/src/components/editor/EditorToolbar.tsx

@@ -1,6 +1,7 @@
 import { useState, useCallback, useRef, useEffect } from 'react';
 import { useTranslation } from 'react-i18next';
 import { registerEditorCommand } from '../../lib/editorCommands';
+import { publishCompileOutput } from '../../lib/intellisenseRegistry';
 import { useEditorStore, chipFileGroupId } from '../../store/useEditorStore';
 import { useSimulatorStore, piRerunScript } from '../../store/useSimulatorStore';
 import { decideEngine } from '../../lib/instantEngine';
@@ -594,6 +595,13 @@ export const EditorToolbar = ({
       const resultLogs = parseCompileResult(result, boardLabel, boardTarget, lastStreamedLen > 0);
       setCompileLogs((prev: CompilationLog[]) => [...prev, ...resultLogs]);
 
+      // Intellisense seam: hand the raw compiler text to the overlay so it
+      // can paint file:line markers in the editor. Empty string clears the
+      // markers after a successful build. No-op in OSS.
+      publishCompileOutput(
+        result.success ? '' : [result.stderr, result.error].filter(Boolean).join('\n'),
+      );
+
       if (result.success) {
         const program = result.hex_content ?? result.binary_content ?? null;
         if (program && activeBoardId) {

frontend/src/lib/intellisenseRegistry.ts

@@ -43,3 +43,27 @@ function run(impl: IntellisenseImpl, monaco: unknown): void {
     console.warn('[oss] intellisense impl threw:', err);
   }
 }
+
+/**
+ * Compile-diagnostics seam. The editor toolbar publishes the raw compiler
+ * output after every build (empty string on success, to clear); with the
+ * pro overlay loaded, its sink parses gcc/mpy positions out of the text
+ * and paints Monaco markers on the affected lines. Inert in OSS.
+ */
+
+type CompileOutputSink = (raw: string) => void;
+
+let _compileSink: CompileOutputSink | null = null;
+
+export function installCompileDiagnosticsSink(sink: CompileOutputSink | null): void {
+  _compileSink = sink;
+}
+
+export function publishCompileOutput(raw: string): void {
+  try {
+    _compileSink?.(raw);
+  } catch (err) {
+    // eslint-disable-next-line no-console
+    console.warn('[oss] compile-diagnostics sink threw:', err);
+  }
+}