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/src/lib.rs

@@ -170,9 +170,16 @@ pub trait RemoteBackend: Send + Sync {
     /// agreed to. **Never for filling a grid**: doing so downloads the entire
     /// library to produce thumbnails.
     ///
-    /// Returns once the content is readable. Callers that borrowed it should
-    /// hand it back with [`dematerialise`](Self::dematerialise).
-    async fn materialise(&self, _id: &RemoteId) -> Result<(), RemoteError> {
+    /// Returns once the content is readable, and **whether this call is what
+    /// brought it here** — `false` meaning it was already local.
+    ///
+    /// That boolean is the whole basis of borrowing. A caller releasing what
+    /// it fetched must not release what the user already had, and after the
+    /// fact the two are indistinguishable; the backend knows because it had to
+    /// look before deciding whether to ask. Answering it here costs the `stat`
+    /// the implementation performs anyway, where a caller determining it
+    /// separately would pay a directory listing per file.
+    async fn materialise(&self, _id: &RemoteId) -> Result<bool, RemoteError> {
         Err(RemoteError::Unsupported(
             "this backend has no placeholders to materialise",
         ))