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/capability.rs

@@ -38,6 +38,50 @@ pub struct ChunkConstraints {
     pub max_chunks: u32,
 }
 
+/// TRACES: FR-NC-6c
+/// Whether every listed object's content is actually reachable.
+///
+/// Every backend but a virtual-filesystem folder answers [`Always`](Self::Always).
+/// A VFS folder is the case this exists for: the sync client leaves a
+/// placeholder where a file is catalogued but not downloaded, so the name is
+/// listable and the bytes are not (ARCH §9.0).
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Materialisation {
+    /// Listing an object means its content can be read. Every server backend,
+    /// and a plain directory.
+    Always,
+
+    /// Some objects are placeholders, and nothing this process can do will
+    /// change that — the sync client is not running, or the platform offers no
+    /// way to ask. Such an object reads as
+    /// [`RemoteError::NotMaterialised`](crate::RemoteError::NotMaterialised)
+    /// and is shown as offline rather than broken.
+    Placeholders,
+
+    /// Some objects are placeholders, and this backend can ask for their
+    /// content — and give it back.
+    ///
+    /// **Whole-file, and that is the whole difficulty.** Hydration has two
+    /// states, one byte or all bytes, so using it to fill a grid transfers the
+    /// entire library to produce thumbnails (ARCH §9.0). It belongs to the
+    /// originals tier — an image opened in develop, exported, or deliberately
+    /// pinned — and to passes the user has asked for and been quoted a price
+    /// on. Never to browsing.
+    OnDemand,
+}
+
+impl Materialisation {
+    /// Whether content can be fetched on request.
+    pub fn can_materialise(self) -> bool {
+        matches!(self, Materialisation::OnDemand)
+    }
+
+    /// Whether some objects may have no content locally.
+    pub fn has_placeholders(self) -> bool {
+        !matches!(self, Materialisation::Always)
+    }
+}
+
 /// TRACES: FR-NC-3
 /// Whether the server can render thumbnails, and for what.
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
@@ -69,6 +113,8 @@ pub struct Capabilities {
     /// Conditional write (If-Match), for conflict-safe sidecar updates.
     pub conditional_write: bool,
     pub server_previews: ServerPreviews,
+    /// Whether a listed object's content is necessarily present.
+    pub materialisation: Materialisation,
 }
 
 impl Capabilities {
@@ -83,6 +129,9 @@ impl Capabilities {
             bulk_upload: false,
             conditional_write: false,
             server_previews: ServerPreviews::None,
+            // The weakest backend still answers for everything it lists;
+            // placeholders are a property a backend opts into.
+            materialisation: Materialisation::Always,
         }
     }