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/remote.rs

@@ -25,7 +25,7 @@ use std::sync::{Arc, OnceLock};
 
 use dr_sync::{Account, BackendProvider, BackendRegistry, Connection, RemoteBackend, RemoteError};
 use dr_sync_folder::FolderProvider;
-use dr_sync_nextcloud::NextcloudProvider;
+use dr_sync_nextcloud::{NextcloudProvider, NextcloudVfs};
 
 /// Every storage backend this build has, in the order the launch screen
 /// offers them.
@@ -37,7 +37,16 @@ pub fn registry() -> &'static BackendRegistry {
     REGISTRY.get_or_init(|| {
         let mut r = BackendRegistry::new();
         r.register(Arc::new(NextcloudProvider));
-        r.register(Arc::new(FolderProvider));
+        // TRACES: FR-NC-6c
+        // The folder connector does the filesystem work and knows nothing
+        // about sync clients; the placeholder convention is supplied here,
+        // which is the one place that may name one. Detection is per folder
+        // and per connection — the same directory offers hydration while the
+        // client is running and not while it is down (ARCH §9.0).
+        r.register(Arc::new(FolderProvider::with_vfs_detector(|root| {
+            NextcloudVfs::looks_synced(root)
+                .then(|| Arc::new(NextcloudVfs::detect()) as Arc<dyn dr_sync_folder::Vfs>)
+        })));
         r
     })
 }