Synchronize Folder: progress reaches its end; say what still writes on the UI thread (review fixes)

The relink check counted only the photos still to relink, so when some
moved files were gone again the bar stopped short of its total. The
module docs now name the writes that stay on the UI thread after each
step (the durable save; auto-written sidecars), and a test that only
showed the work checks files again is named for that.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/sync.rs

@@ -16,7 +16,9 @@
 //! [`synchronize`] then imports the new files (in place), relinks the moved photos, moves the
 //! missing photos to Recently Deleted and reads the metadata updates — each only when chosen — as
 //! one undo step. It runs in halves ([`SyncJob`]) so that, in the app, every file it reads is
-//! read on a worker thread and only the catalog changes happen on the UI thread.
+//! read on a worker thread and only the catalog changes happen on the UI thread — with what every
+//! command does after them there: the library's durable save and, with XMP auto-write on, the
+//! sidecars of the photos whose metadata was read.
 //!
 //! The scan runs without the session ([`SyncInput`] + [`scan_with`]), so the app can run it on a
 //! worker thread: a folder on a sleeping network share must never stall a frame.
@@ -572,9 +574,10 @@ impl SyncWork {
             return;
         }
         // still where the scan found it, and still gone from where the photo says
+        let checked = moved.len();
         let relink: Vec<(PhotoId, String)> =
             moved.into_iter().filter(|m| Path::new(&m.to).is_file() && !Path::new(&m.from).exists()).map(|m| (PhotoId(m.id), m.to)).collect();
-        step(relink.len());
+        step(checked);
         if !relink.is_empty() && !send(SyncStep::Relink(relink)) {
             return;
         }

crates/engine/src/tests_sync.rs

@@ -498,7 +498,7 @@ fn the_work_of_synchronizing_runs_apart_from_its_commits() {
 }
 
 #[test]
-fn readying_a_synchronize_reads_no_file() {
+fn the_work_checks_again_that_a_new_file_is_still_there() {
     let dir = Scratch::new("ready");
     let mut s = library(&dir);
     write_png(&dir.path("trip/c.png"), 3);