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/read.test.ts

@@ -531,7 +531,7 @@ describe("Library exposes the read surface over its live store", () => {
             expect(client.collectionsCalls).toBe(collectionsBefore);
             expect(client.filesCalls).toBe(filesBefore);
         } finally {
-            lib.close();
+            await lib.close();
         }
     });
 });