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

test/library/snapshot.test.ts

@@ -159,7 +159,7 @@ describe("Library.snapshot and Library.subscribe", () => {
             for (const p of snap.photos) expect("key" in p).toBe(false);
             for (const a of snap.albums) expect("key" in a).toBe(false);
         } finally {
-            lib.close();
+            await lib.close();
         }
     });
 
@@ -219,7 +219,7 @@ describe("Library.snapshot and Library.subscribe", () => {
             expect(change.refreshedAt).toBeGreaterThan(0);
         } finally {
             unsubscribe();
-            lib.close();
+            await lib.close();
         }
     });
 
@@ -251,7 +251,7 @@ describe("Library.snapshot and Library.subscribe", () => {
             expect(changes).toEqual([]);
         } finally {
             unsubscribe();
-            lib.close();
+            await lib.close();
         }
     });
 
@@ -297,7 +297,7 @@ describe("Library.snapshot and Library.subscribe", () => {
             );
             expect(changes).toEqual([]);
         } finally {
-            lib.close();
+            await lib.close();
         }
     });
 });