feat(gateway): MosaicPlugin lifecycle + ReloadService + hot reload (P8-013) (#182)

Co-authored-by: Jason Woltje <jason@diversecanvas.com>
Co-committed-by: Jason Woltje <jason@diversecanvas.com>

apps/gateway/src/reload/mosaic-plugin.interface.ts

@@ -0,0 +1,20 @@
+export interface MosaicPlugin {
+  /** Called when the plugin is loaded/reloaded */
+  onLoad(): Promise<void>;
+
+  /** Called before the plugin is unloaded during reload */
+  onUnload(): Promise<void>;
+
+  /** Plugin identifier for registry */
+  readonly pluginName: string;
+}
+
+export function isMosaicPlugin(obj: unknown): obj is MosaicPlugin {
+  return (
+    typeof obj === 'object' &&
+    obj !== null &&
+    typeof (obj as MosaicPlugin).onLoad === 'function' &&
+    typeof (obj as MosaicPlugin).onUnload === 'function' &&
+    typeof (obj as MosaicPlugin).pluginName === 'string'
+  );
+}