Add folder scan with format selection; validate A3 on a real library

Library setup as the user described it: pick a folder, choose which RAW
types to look for, scan recursively.

  dr-types::FormatFilter  the tick-box selection, seeing through VFS
                          placeholder suffixes so a dehydrated CR2 still
                          matches as a CR2
  dr-sync::scan           recursive walk, Depth:1 per directory, pruning
                          unchanged subtrees where the backend propagates
                          directory ETags

Verified against nextcloud.tourolle.paris (34.0.2) on a real library:

  browse root      32 entries, 98ms
  scan PhotosRaw   17,185 RAW files in 334 directories, 34.1s
                   (7,836 CR2 + 9,349 DNG)
  range read       262KB of a 21.5MB DNG in 119ms — 1.22% of the file,
                   and enough to read "Canon EOS 6D | ISO 100"

That last line is assumption A3 validated on real data. Cataloguing this
library by whole-file fetch would move roughly 370GB; the range path
moves a few MB.

Pruning is capability-gated rather than assumed: with per-entry ETags a
probe costs a request and proves nothing about children, so it is skipped
entirely. A test asserts zero probes in that case.

Still unresolved: /core/preview returns 400 for every parameter
combination tried, including on a JPEG the server reports as having a
preview. Not a request-shape bug — it fails identically bare. Recorded
rather than worked around; ARCH §6.7 already treats server previews as
opportunistic, so nothing depends on it.

core/dr-sync/src/lib.rs

@@ -22,10 +22,12 @@ use async_trait::async_trait;
 
 pub mod capability;
 pub mod error;
+pub mod scan;
 pub mod types;
 
 pub use capability::{Capabilities, ChangeDetection, ChunkConstraints, ServerPreviews};
 pub use error::RemoteError;
+pub use scan::{scan, ScanProgress, ScanResult};
 pub use types::{
     Cursor, EntryKind, Identity, Precondition, RemoteChange, RemoteEntry, RemoteId, RemotePath,
     Validator,