Treat a placeholder as the photograph, not as a one-byte file

The folder connector was pointed at a Nextcloud VFS tree and got three
things wrong, the first of which loses work.

**A dehydrated sidecar read as absent.** `a.drsc` does not exist when the
client has dehydrated it — only `a.drsc.nextcloud` does — so `get` missed,
`.ok()` swallowed the `NotFound`, and the sidecar writer took that for
"there is no sidecar yet" and wrote a fresh document over the existing
one. Every edit another device had put there went with it. That function's
own doc comment calls this the exact loss the format's unknown-key
preservation exists to prevent.

**A stub was catalogued as a 1-byte image**, and ARCH §9.0 measured this
machine at 121,785 placeholders against 10,267 real files — so a folder
library on a synced tree was ~92% broken rows.

**Identity changed on hydration**, so downloading a photograph looked like
a delete and an add, orphaning its thumbnail and its face rows.

Entries now carry the photograph's own name and a `materialised` flag;
`get` on a stub returns the new `RemoteError::NotMaterialised`, which is
distinct from `NotFound` precisely because the sidecar writer must treat
them differently — it fetches the sidecar and merges, or leaves the entry
queued.

Hydration is a **borrow**. `BorrowPool` records what was on disk before it
asked, so `release_all` dehydrates only what a pass brought and leaves
what the user already had. Reference counted: the thumbnail pass and the
face pass meet on the same RAW, and without counting the first to finish
dehydrates the file the second is reading. A borrow against a plain folder
or a server does nothing, so a pass written for VFS runs everywhere.

Releasing means asking the client to dehydrate and never deleting: a
deletion inside a synced tree propagates to the server and removes the
photograph from every device.

Not a second backend — the capability is per *connection*, not per type,
since the same folder hydrates only while the client runs. The convention
arrives through a detector the registry supplies, so `dr-sync-folder`
still knows nothing about any client's protocol.

ARCH §9.0a records this as an amendment: finding 3 rejected hydration
because it costs 100× a range read, and that comparison assumed a
connector was available. A folder library has none.

core/dr-sync/src/lib.rs

@@ -35,7 +35,9 @@ pub mod types;
 pub mod upload;
 
 pub use account::{Account, AccountError, AccountStore, Connection, Secret, LEGACY_BACKEND};
-pub use capability::{Capabilities, ChangeDetection, ChunkConstraints, ServerPreviews};
+pub use capability::{
+    Capabilities, ChangeDetection, ChunkConstraints, Materialisation, ServerPreviews,
+};
 pub use error::RemoteError;
 pub use provider::{BackendProvider, BackendRegistry, SignIn};
 pub use reachability::{Connectivity, Reachability};
@@ -151,6 +153,49 @@ pub trait RemoteBackend: Send + Sync {
     /// destination, not to claim they created it.
     async fn create_dir(&self, path: &RemotePath) -> Result<(), RemoteError>;
 
+    // ---- materialisation --------------------------------------------------
+
+    /// TRACES: FR-NC-6c
+    /// Ask for a placeholder's content to be brought to this device.
+    ///
+    /// Only meaningful where [`Capabilities::materialisation`] is
+    /// [`Materialisation::OnDemand`]; others return
+    /// [`RemoteError::Unsupported`].
+    ///
+    /// **Whole-file, and slow.** There is no partial hydration: a placeholder
+    /// becomes one byte or all of them, so this transfers a 27 MB RAW to
+    /// answer a question a 256 KB range read would have answered (ARCH §9.0
+    /// finding 3). It is for the originals tier — develop, export, a pin the
+    /// user asked for — and for passes the user has been quoted a price on and
+    /// 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> {
+        Err(RemoteError::Unsupported(
+            "this backend has no placeholders to materialise",
+        ))
+    }
+
+    /// Give a placeholder's content back, freeing the disk it held.
+    ///
+    /// The counterpart that makes hydration a *borrow* rather than an
+    /// acquisition: a pass that hydrates a library to index it can return each
+    /// file as it finishes, so peak disk is the working set rather than the
+    /// library.
+    ///
+    /// **Never destructive.** On a synced folder this asks the client to
+    /// dehydrate; it must not delete, because a deletion in a synced tree
+    /// propagates to the server and removes the photograph everywhere. An
+    /// implementation that cannot dehydrate must return
+    /// [`RemoteError::Unsupported`] rather than approximating it.
+    async fn dematerialise(&self, _id: &RemoteId) -> Result<(), RemoteError> {
+        Err(RemoteError::Unsupported(
+            "this backend has no placeholders to release",
+        ))
+    }
+
     // ---- optional ---------------------------------------------------------
 
     /// Server-rendered thumbnail, where available.