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.

ui/dr-ui/src/library.rs

@@ -25,7 +25,7 @@ use std::path::PathBuf;
 use std::sync::mpsc::{Receiver, Sender};
 
 use dr_catalog::{Catalog, JobKind, Priority};
-use dr_sync::{Account, Connection, RemoteBackend, RemoteId, RemotePath};
+use dr_sync::{Account, Connection, RemoteBackend, RemoteError, RemoteId, RemotePath};
 use dr_thumbs::ThumbStore;
 
 use crate::sidecar_cache::SidecarCache;
@@ -944,7 +944,35 @@ async fn drain_one(
 
     let path = RemotePath::new(path_str.to_string());
     let id = RemoteId::Path(path.clone());
-    let remote = backend.get(&id, None).await.ok();
+
+    // TRACES: FR-NC-6c
+    // A miss and a placeholder are not the same answer, and conflating them
+    // destroys work. This read decides whether the sidecar already on the
+    // remote is merged in; treating "the content is not on this device" as
+    // "there is no sidecar" writes a fresh document over an existing one and
+    // discards every edit another device put there — the exact loss the
+    // format's unknown-key preservation exists to prevent.
+    //
+    // A sidecar is a few kilobytes, so the right response to a placeholder is
+    // to fetch it, not to give up. Where that is impossible — no client
+    // running — the entry stays queued, which is what the outbox is for.
+    let remote = match backend.get(&id, None).await {
+        Ok(bytes) => Some(bytes),
+        Err(RemoteError::NotFound(_)) => None,
+        Err(RemoteError::NotMaterialised(_)) => {
+            backend
+                .materialise(&id)
+                .await
+                .map_err(|e| format!("sidecar is not on this device ({e})"))?;
+            match backend.get(&id, None).await {
+                Ok(bytes) => Some(bytes),
+                Err(e) => return Err(format!("sidecar could not be read ({e})")),
+            }
+        }
+        // Anything else — a refused read, a dead connection — leaves the entry
+        // queued rather than resolved by overwriting.
+        Err(e) => return Err(format!("sidecar could not be read ({e})")),
+    };
 
     if let Some(bytes) = remote.as_deref() {
         if !bytes.is_empty() {
@@ -5440,6 +5468,7 @@ mod tests {
             size,
             modified: None,
             has_preview: false,
+            materialised: true,
         }
     }