Borrow the library to index it, and give it back

The passes that need every photograph's bytes — thumbnails, face
indexing — now borrow each one and release it at the end. On a
placeholder library that is the difference between peak disk being the
working set and being the whole library.

Including on cancellation, which was nearly missed: the face sweep
returns mid-loop when the user presses Stop, and without releasing there
the disk is spent and nothing is delivered for it.

`materialise` now answers whether *it* fetched the content. The pool used
to work that out by listing a file's parent directory — one listing per
file across a library — when the backend already had to `stat` it to
decide whether to ask. One syscall instead of a directory walk, and it
removes the bug class the tests found earlier: a file at the library root
has no `parent()`, so every one of them read as already-downloaded.

**Pinning is the retention control**, and it drives the model the catalog
already had rather than a second one. `tier_desired` is what the user
asked to keep hydrated, `pending_pins` is the resumable work list, and a
pinned collection is never dehydrated for the same reason it was never
evicted. It was in fact *broken* here before: `get` on a stub failed, and
the pin worker logged "one unreadable file must not abandon the whole
pin" and silently did nothing.

Pinned originals on such a library are recorded with `path = NULL`
(`Cache::record_in_place`) rather than copied under `originals/`. Two
reasons, and the second is the important one. A copy would hold every
pinned photograph twice, with the budget able to evict the half that was
not costing the disk. And `release` deletes the file a row names — so a
row that names none cannot delete anything, which puts the one
catastrophic operation out of reach by construction rather than by
remembering not to call it. Deleting a materialised file inside a synced
tree removes the photograph from the server and every other device.

Handing disk back is `spawn_dehydrate`, which asks the client.

Two gaps written down rather than papered over (docs/storage.md §7): a
hydrating pass cannot yet quote its cost, because a stub reports no size;
and the two sweeps hold separate pools, so a library indexed for both
fetches twice.

core/dr-sync-folder/src/borrow/tests.rs

@@ -221,22 +221,46 @@ async fn borrowing_against_a_plain_folder_does_nothing_at_all() {
 }
 
 #[tokio::test]
-async fn an_uncertain_caller_keeps_the_file_rather_than_releasing_it() {
-    // The asymmetry stated on `borrow_known`: claiming "already local" costs
-    // disk, claiming "not local" can release a pin.
-    let t = Tmp::new("uncertain");
-    t.real("a.CR2");
+async fn the_backend_is_what_decides_whether_a_file_was_ours() {
+    // Not the pool, and not the caller. The backend had to look before
+    // deciding whether to ask, so it can answer for the cost of that same
+    // `stat`; a borrower working it out separately would pay a directory
+    // listing per file and could get it wrong in the direction that releases
+    // a file the user pinned.
+    let t = Tmp::new("who-decides");
+    t.real("had.CR2").stub("wanted.CR2");
+    let b = t.backend(FakeClient::new());
+
+    assert!(
+        !b.materialise(&RemoteId::Path(RemotePath::new("had.CR2")))
+            .await
+            .unwrap(),
+        "already here, so not ours to release"
+    );
+    assert!(
+        b.materialise(&RemoteId::Path(RemotePath::new("wanted.CR2")))
+            .await
+            .unwrap(),
+        "this call fetched it"
+    );
+}
+
+#[tokio::test]
+async fn a_file_borrowed_twice_in_one_pass_is_fetched_once_and_released_once() {
+    // Thumbnailing and face indexing visit the same photograph. Fetching it
+    // per stage doubles the transfer over the whole library.
+    let t = Tmp::new("sequential");
+    t.stub("a.CR2");
     let client = FakeClient::new();
     let b = t.backend(client.clone());
     let pool = BorrowPool::new();
+    let path = RemotePath::new("a.CR2");
 
-    {
-        let _held = pool
-            .borrow_known(&b, &RemotePath::new("a.CR2"), Some(true))
-            .await
-            .unwrap();
-    }
-    pool.release_all(&b).await;
-    assert!(t.has("a.CR2"), "kept");
-    assert_eq!(client.dehydrations.load(Ordering::SeqCst), 0);
+    // Sequential borrows, as two passes over one work list would make.
+    drop(pool.borrow(&b, &path).await.unwrap());
+    drop(pool.borrow(&b, &path).await.unwrap());
+
+    assert_eq!(client.hydrations.load(Ordering::SeqCst), 1, "paid once");
+    let stats = pool.release_all(&b).await;
+    assert_eq!(stats.released, 1, "given back once");
 }