refactor: let each store bring its own step for the shared-store import

lib/shared-store/import.ts

@@ -2,12 +2,21 @@ import type { ImportSummary } from './boot';
 
 /**
  * Copy an existing single-instance install's local files into an
- * uninitialised shared store (#1066). Every store adds its own step here;
- * each step writes only what the store doesn't hold yet (put with
- * ifVersion: null), so running it again changes nothing, and it never
- * changes or deletes a local file.
+ * uninitialised shared store (#1066). Every store brings its own steps (in
+ * lib/shared-store/import/<part>.ts); each step reads that store's local
+ * files exactly as local mode would and writes only what the store doesn't
+ * hold yet (put with ifVersion: null), so running it again changes nothing.
+ * It never changes or deletes a local file. Returns how many items it wrote.
  */
-export async function importLocalData(): Promise<ImportSummary> {
+export interface ImportStep {
+  name: string;
+  run(): Promise<number>;
+}
+
+const STEPS: ImportStep[] = [];
+
+export async function importLocalData(steps: ImportStep[] = STEPS): Promise<ImportSummary> {
   const imported: Record<string, number> = {};
+  for (const step of steps) imported[step.name] = await step.run();
   return { imported };
 }