Shorten internal/vaultik tests under the race detector (closes #416)

All SQLite work in the package serializes on one allocator lock under
-race, so its run time follows the total work. Tests that need one
backup to a file:// destination now copy a shared backup made once.
The sweeper and locality fixtures are smaller, and the list truncation
test encodes one manifest instead of 1001, which cut the part of the
run that is not parallel from about 11 to 3 seconds.

Model: opus-5-5

docs/TODO.md

@@ -26,7 +26,7 @@ runs the release build in `.gitea/workflows/release.yml`.
 - 2026-11-10: Shortened the tests of `internal/vaultik` under the race detector,
   which on a busy host ran past the 90-second test timeout
   ([issue #416](https://git.eeqj.de/sneak/vaultik/issues/416)). Tests that need
-  one backup to a `file://` destination now copy a shared one, and the largest
+  one backup to a file:// destination now copy a shared one, and the largest
   fixtures are smaller.
 
 - 2026-11-10: `script/fmt` and `script/fmt-check` now format and check all

internal/vaultik/missing_destination_test.go

@@ -113,10 +113,8 @@ func makeFirstBackup(ctx context.Context, t *testing.T) {
 // before the first backup, and returns a Vaultik whose file://
 // destination there holds a backup of the snapshot named "first". The
 // store and the local index are copies of firstBackup's, which is faster
-// than backing up in every test. The returned Vaultik backs up a source
-// tree shared with every other caller, so no test may change that tree.
-// Everything the returned Vaultik prints after the backup goes to the
-// returned buffer.
+// than backing up in every test. Everything the returned Vaultik prints
+// after the backup goes to the returned buffer.
 func backUpToFileDestination(
 	ctx context.Context, t *testing.T, storeDir string,
 ) (*vaultik.Vaultik, *database.Repositories, *bytes.Buffer) {