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-nextcloud/src/lib.rs

@@ -24,7 +24,7 @@ mod propfind;
 pub mod provider;
 
 pub use auth::{AppCredentials, LoginFlow};
-pub use desktop_client::DesktopClient;
+pub use desktop_client::{DesktopClient, NextcloudVfs};
 pub use provider::NextcloudProvider;
 
 /// Chunk sizes Nextcloud's chunked upload v2 accepts.
@@ -73,6 +73,10 @@ impl NextcloudBackend {
                 // Stock Nextcloud ships no RAW preview provider (ARCH §6.7).
                 // Probed per-account at setup and upgraded where present.
                 server_previews: ServerPreviews::CommonFormatsOnly,
+                // The server answers for everything it lists. Placeholders
+                // belong to a locally *synced folder*, which is the folder
+                // connector's business (`desktop_client::NextcloudVfs`).
+                materialisation: dr_sync::Materialisation::Always,
             },
         })
     }