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 and CLI command now awaits close(), and
tests hold each of the three writes 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.