Fix two intermittently failing library tests (closes #90)

Library.close() now returns a promise that resolves once the work it
started has finished: an in-flight refresh with its cache write, the ML
data fetch, and running precache sweeps. The interval test could see a
refresh's new state, close, and remove the directory while the write was
still running. Every library test now awaits close(), and a new test
holds a cache write open to prove close() waits for it.

The precache test waited for its stub source to be called, but the cache
records a file only after checking it on disk, so status() could lag. It
now waits for both fills to report "done".

Model: opus-5-5

test/library/content-library.test.ts

@@ -116,7 +116,7 @@ describe("Library content wiring", () => {
         expect(lib.photos.byID({ fileID: 1 })!.record().thumbnailPath).toBe(
             result.path,
         );
-        lib.close();
+        await lib.close();
     });
 
     it("drives thumbnails.ensure through the cache", async () => {
@@ -139,7 +139,7 @@ describe("Library content wiring", () => {
         expect(results).toEqual([
             { fileID: 1, path: join(root, "cache", "thumbnails", "1.jpg") },
         ]);
-        lib.close();
+        await lib.close();
     });
 
     it("throws from content methods when opened without a content source", async () => {
@@ -155,6 +155,6 @@ describe("Library content wiring", () => {
         await expect(
             lib.thumbnails.ensure({ fileIDs: [1], priority: "visible" }),
         ).rejects.toThrow(/content cache/i);
-        lib.close();
+        await lib.close();
     });
 });