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

README.md

@@ -73,7 +73,7 @@ if (photo) {
     console.log(`original at ${path}`);
 }
 
-lib.close();
+await lib.close();
 ```
 
 The lower-level `Client` (login, session serialization, and the raw
@@ -563,7 +563,10 @@ and pass it. The three pools default to 10 / 5 / 25 (see Request pools below).
 `lib.status()` returns a `LibraryStatus` (collection/file counts, last
 refresh/ML times and errors, originals usage and effective limit, precache
 progress, and `closed`). `lib.close()` stops the background timer; it is
-idempotent, and an in-flight refresh is left to finish.
+idempotent, and an in-flight refresh is left to finish. The promise it returns
+resolves once that refresh (including its cache write), the ML data fetch and
+the precache fetches already running have all finished, so the cache directory
+can then be removed.
 
 ### Default reads vs. fresh reads