feat(compile): expose compile logs via Zustand store + UI slot

Two minimal hooks so the velxio-pro agent overlay can offer a 'Diagnose
this compile failure with AI' affordance without touching upstream
component internals:

  - New store/useCompileLogsStore: holds the editor's compile output as
    Zustand state instead of local React useState in EditorPage. The
    setter accepts both a value and an updater fn so the EditorToolbar
    callers that used setCompileLogs(prev => [...prev, log]) keep
    working without changes.

  - CompilationConsole header now renders a
    <div data-velxio-slot='compile-console-actions' /> when errorCount
    > 0. The pro overlay mounts a 'Diagnose with AI' button into this
    slot via slotMounter. Empty in the OSS image — no behaviour change.

EditorPage replaces its local useState<CompilationLog[]> with the store
selector. The downstream prop-drilled setCompileLogs callers (toolbar,
sub-toolbars) keep their signature.

Companion commit lands the button + diagnostic prompt builder in the
velxio-prod overlay.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/components/editor/CompilationConsole.tsx

@@ -75,6 +75,12 @@ export const CompilationConsole: React.FC<CompilationConsoleProps> = ({
                 ⚠ {warningCount}
               </span>
             )}
+            {/* Pro overlay mounts a "Diagnose with AI" button here when
+                errorCount > 0. Empty in the OSS image — slotMounter
+                only fires when the pro tree is present. */}
+            {errorCount > 0 && (
+              <div data-velxio-slot="compile-console-actions" />
+            )}
           </div>
         </div>
         <div style={styles.headerRight}>

frontend/src/pages/EditorPage.tsx

@@ -27,6 +27,7 @@ import { LoginPromptModal } from '../components/layout/LoginPromptModal';
 import { GitHubStarBanner } from '../components/layout/GitHubStarBanner';
 import { useSimulatorStore, DEFAULT_BOARD_POSITION } from '../store/useSimulatorStore';
 import { useEditorStore } from '../store/useEditorStore';
+import { useCompileLogsStore } from '../store/useCompileLogsStore';
 import { useOscilloscopeStore } from '../store/useOscilloscopeStore';
 import { useAuthStore } from '../store/useAuthStore';
 import { useProjectStore } from '../store/useProjectStore';
@@ -81,7 +82,11 @@ export const EditorPage: React.FC = () => {
   const isRaspberryPi3 = activeBoardKind === 'raspberry-pi-3';
   const oscilloscopeOpen = useOscilloscopeStore((s) => s.open);
   const [consoleOpen, setConsoleOpen] = useState(false);
-  const [compileLogs, setCompileLogs] = useState<CompilationLog[]>([]);
+  // compileLogs live in a Zustand store so the velxio-pro agent overlay
+  // (mounted in a separate React tree via slotMounter) can subscribe and
+  // build a "diagnose this failure" prompt without prop-drilling.
+  const compileLogs = useCompileLogsStore((s) => s.logs);
+  const setCompileLogs = useCompileLogsStore((s) => s.setLogs);
   const [bottomPanelHeight, setBottomPanelHeight] = useState(BOTTOM_PANEL_DEFAULT);
   const [saveModalOpen, setSaveModalOpen] = useState(false);
   const [loginPromptOpen, setLoginPromptOpen] = useState(false);

frontend/src/store/useCompileLogsStore.ts

@@ -0,0 +1,42 @@
+/**
+ * Zustand store for the editor's compile output.
+ *
+ * Lives outside <EditorPage> so other components (notably the velxio-pro
+ * agent overlay, mounted into a different React tree via slotMounter) can
+ * subscribe without prop-drilling.  The overlay reads `logs` to build a
+ * "diagnose this compile failure with AI" prompt; without the store it
+ * would have no way to reach the upstream component state.  Board target
+ * is read from `useEditorStore` by the overlay independently.
+ */
+
+import { create } from 'zustand';
+
+import type { CompilationLog } from '../utils/compilationLogger';
+
+/** React-style setter — accepts either a new value OR an updater fn so
+ *  callers that used `useState`'s `setX(prev => ...)` form can be
+ *  swapped in without rewriting the call sites. */
+type LogsSetter = (
+  next: CompilationLog[] | ((prev: CompilationLog[]) => CompilationLog[]),
+) => void;
+
+interface CompileLogsState {
+  logs: CompilationLog[];
+  setLogs: LogsSetter;
+  appendLogs: (logs: CompilationLog[]) => void;
+  clear: () => void;
+}
+
+export const useCompileLogsStore = create<CompileLogsState>((set, get) => ({
+  logs: [],
+  setLogs: (next) => {
+    if (typeof next === 'function') {
+      set({ logs: (next as (prev: CompilationLog[]) => CompilationLog[])(get().logs) });
+    } else {
+      set({ logs: next });
+    }
+  },
+  appendLogs: (entries) =>
+    set((s) => ({ logs: [...s.logs, ...entries] })),
+  clear: () => set({ logs: [] }),
+}));