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/error.rs

@@ -36,6 +36,22 @@ pub enum RemoteError {
     #[error("not found: {0}")]
     NotFound(String),
 
+    /// TRACES: FR-NC-6c
+    /// The object exists, but its content is not on this device.
+    ///
+    /// A virtual-filesystem placeholder: the sync client holds the name and a
+    /// stub, and the bytes are still on the server (ARCH §9.0).
+    ///
+    /// **Emphatically not [`NotFound`](Self::NotFound), and the distinction is
+    /// what stops a silent data loss.** The sidecar writer reads before it
+    /// writes, and treats a miss as "there is no sidecar yet, create one" — so
+    /// a dehydrated sidecar reported as absent makes it write a fresh document
+    /// over an existing one, discarding every edit another device had put
+    /// there. It is also the difference between an error a user can act on
+    /// (fetch it) and one they cannot (it is gone).
+    #[error("not on this device: {0}")]
+    NotMaterialised(String),
+
     /// The backend does not support this operation. Expected, not a bug —
     /// callers check capabilities and adapt.
     #[error("operation unsupported by this backend: {0}")]