feat(site-builder): add LayerFocusContext for layers-to-array-editor focus

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

craft/src/panels/left/LayerFocusContext.tsx

@@ -0,0 +1,44 @@
+import React, { createContext, useCallback, useContext, useMemo, useRef, useState } from 'react';
+
+/**
+ * Carries "the user clicked a virtual Layers row -- scroll that array item's
+ * card into view" from the Layers panel (left) to the array editors (right).
+ *
+ * Deliberately a request, not a command: consumers that don't implement
+ * scrolling simply ignore it. Selecting the parent node always happens in
+ * LayersPanel itself, so a click is useful even with no consumer at all.
+ */
+
+export interface LayerFocusRequest {
+  nodeId: string;
+  prop: string;
+  index: number;
+  /** Bumped on every request so an identical repeat still re-fires effects. */
+  nonce: number;
+}
+
+export interface LayerFocusValue {
+  focus: LayerFocusRequest | null;
+  requestFocus(nodeId: string, prop: string, index: number): void;
+}
+
+const LayerFocusContext = createContext<LayerFocusValue>({
+  focus: null,
+  requestFocus: () => {},
+});
+
+export const useLayerFocus = (): LayerFocusValue => useContext(LayerFocusContext);
+
+export const LayerFocusProvider: React.FC<{ children: React.ReactNode }> = ({ children }) => {
+  const [focus, setFocus] = useState<LayerFocusRequest | null>(null);
+  const nonceRef = useRef(0);
+
+  const requestFocus = useCallback((nodeId: string, prop: string, index: number) => {
+    nonceRef.current += 1;
+    setFocus({ nodeId, prop, index, nonce: nonceRef.current });
+  }, []);
+
+  const value = useMemo(() => ({ focus, requestFocus }), [focus, requestFocus]);
+
+  return <LayerFocusContext.Provider value={value}>{children}</LayerFocusContext.Provider>;
+};