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

TODO.md

@@ -18,6 +18,13 @@ Tag v1.0.0.
 
 # Completed Steps
 
+- 2026-09-23: Fixed two intermittently failing library tests (issue 90).
+  `Library.close()` now returns a promise that resolves once an in-flight
+  refresh (including its cache write), the ML data fetch and running precache
+  sweeps have finished; the library tests await it, so `afterEach` no longer
+  removes the cache directory while something is still writing into it. The
+  precache test waits for both fills to report "done" instead of for its stub
+  source to be called, which happened before the cache recorded the file.
 - 2026-09-23: Pinned three guards reviewers found untested (issue 89). The
   download idle deadline's timer is unref'd, so it can never keep the process
   alive, and a test checks no timer is left after a download completes or fails.