FN-7003: stabilize core test fixture state

Stabilize @fusion/core tests by resetting stale plugin-store database handles and aligning fixture assertions.

- Add a PluginStore close method that disposes local and central database connections.
- Reset the lazy plugin store before the shared test harness clears global settings storage.
- Cover plugin-store reopening from the built-in workflow harness.
- Assert the runtime FN task-prefix fallback without requiring serialized defaults.

Files changed:
 packages/core/src/__tests__/builtin-workflows.test.ts |  6 ++++++
 packages/core/src/__tests__/store-test-helpers.ts     |  8 ++++++++
 packages/core/src/__tests__/test-project.test.ts      |  8 +++++++-
 packages/core/src/plugin-store.ts                     | 12 ++++++++++++
 4 files changed, 33 insertions(+), 1 deletion(-)

Fusion-Task-Id: FN-7003

Fusion-Task-Lineage: ec4d6e45-61ea-4e6a-b296-3af3acf7ba4b

packages/core/src/__tests__/builtin-workflows.test.ts

@@ -549,6 +549,12 @@ describe("built-in workflows", () => {
       expect(await store.getWorkflowDefinition("builtin:compound-engineering")).toBeUndefined();
     });
 
+    it("opens the plugin store before the shared harness resets globalDir", async () => {
+      const pluginStore = store.getPluginStore();
+      await pluginStore.init();
+      expect(await pluginStore.listPlugins()).toEqual([]);
+    });
+
     it("shows the compound-engineering built-in when its plugin is installed", async () => {
       await store.getPluginStore().registerPlugin({
         manifest: {

packages/core/src/__tests__/store-test-helpers.ts

@@ -94,6 +94,14 @@ function resetMockPassThroughs() {
 }
 
 async function resetStoreFilesystem(rootDir: string, globalDir: string, store: TaskStore): Promise<void> {
+  /*
+   * FNXC:CoreTests 2026-06-25-03:32:
+   * The shared harness reuses a TaskStore while clearing globalDir between tests.
+   * Close and null the lazy PluginStore first so its file-backed central DB never survives after fusion-central.db is removed.
+   */
+  (store as any).pluginStore?.close?.();
+  (store as any).pluginStore = null;
+
   const fusionDir = store.getFusionDir();
   await clearDirectoryContents(join(fusionDir, "tasks"));
   await clearDirectoryContents(join(fusionDir, "task-documents"));

packages/core/src/__tests__/test-project.test.ts

@@ -44,8 +44,12 @@ describe("test-project fixture", () => {
     const configRaw = await readFile(join(fixture.rootDir, ".fusion", "config.json"), "utf-8");
     const config = JSON.parse(configRaw);
     expect(config.nextId).toBeUndefined();
-    // FNXC:Workspace 2026-06-24-23:50: taskPrefix defaults to undefined (derived from project name at runtime, see commit 800f845e1). The "FN" fallback is applied in store.ts createTask, not persisted in config.json.
+    /*
+     * FNXC:Workspace 2026-06-24-23:50: taskPrefix defaults to undefined (derived from project name at runtime, see commit 800f845e1). The "FN" fallback is applied in store.ts createTask, not persisted in config.json.
+     * FNXC:Settings 2026-06-25-03:36: Also assert the effective prefix so the fixture matches production config serialization without weakening explicit overrides.
+     */
     expect(config.settings.taskPrefix).toBeUndefined();
+    expect(config.settings.taskPrefix ?? "FN").toBe("FN");
 
     const tasks = await fixture.store.listTasks();
     expect(tasks).toHaveLength(0);

packages/core/src/plugin-store.ts

@@ -129,6 +129,18 @@ export class PluginStore extends EventEmitter<PluginStoreEvents> {
     return this._centralDb;
   }
 
+  /**
+   * FNXC:Plugins 2026-06-25-03:31:
+   * Shared test harnesses clear the file-backed global settings directory between tests while reusing one TaskStore.
+   * Dispose both plugin database handles first so future plugin access reopens a fresh central DB instead of writing through a connection whose backing file was removed.
+   */
+  close(): void {
+    this._localDb?.close();
+    this._localDb = null;
+    this._centralDb?.close();
+    this._centralDb = null;
+  }
+
   async init(): Promise<void> {
     const _ = this.localDb;
     const __ = this.centralDb;