feat(app): load xterm search addon + surface→addon registry

app/src/searchRegistry.ts

@@ -0,0 +1,17 @@
+import type { SearchAddon } from "@xterm/addon-search";
+
+/** Maps a surfaceId to its terminal's SearchAddon so the search bar can reach
+ * the focused panel without prop-drilling through the layout tree. */
+const registry = new Map<string, SearchAddon>();
+
+export function registerSearch(surfaceId: string, addon: SearchAddon): void {
+  registry.set(surfaceId, addon);
+}
+
+export function unregisterSearch(surfaceId: string): void {
+  registry.delete(surfaceId);
+}
+
+export function getSearch(surfaceId: string): SearchAddon | undefined {
+  return registry.get(surfaceId);
+}