Read the sidecars other editors write, and write them back on request

FR-CAT-13 asked for standard XMP and `core/dr-xmp` answered the file: it
has read and written `dc:subject`, `xmp:Rating`, `xmp:Label` and the IPTC
core since 5fa4c07, under an ownership rule that leaves everything else in
the document untouched. What nothing did was call it. No scan found an
`.xmp` beside a raw, no catalog row was filled from one, no judgement
wrote one back, and the "external modification detected, reload offered"
clause had no mechanism. A library imported from Lightroom came in and
could not go back out.

The scan collects `.xmp` beside `.drsc` from the listings it was already
paying for, and the pull reads each one whose ETag has moved. Both
namings resolve: darktable's `IMG_0001.CR3.xmp` names its file exactly,
Lightroom's `IMG_0001.xmp` names the stem, and under the stem the JPEG
beside a RAW is the same photograph and takes the same document, as
DarkRoom's own sidecar already does. Each is reconciled with the catalog
winning — keywords union, a rating or label taken only where the catalog
has none — because a standard XMP carries nothing that could say whether
its value is newer. A genuine disagreement is not resolved; it is written
to a table, and the settings page offers the sidecars' values against it.
That button is the reload the requirement asks to be offered, and the
ETag that moved is the detection it asks for: an `.xmp` edited elsewhere
is exactly a file the pull's ordinary incrementality re-reads.

Writing goes the other way behind a setting that starts off, since NFR-R4
makes writes beside somebody's originals theirs to switch on. With it on,
a judgement or a keyword rewrites the sidecar of whichever spelling
exists, or creates Lightroom's. The record is read from the catalog
whole at that moment rather than carried from the gesture, so a rating
and a keyword a second apart are two writes of one file that agree. And
the file's own title, caption, copyright and hierarchy come through the
rewrite: the catalog has no columns for them, `rewrite` replaces the
owned set wholesale, and a record that said nothing about them would have
deleted them from a Lightroom sidecar on every star.

The rating's two axes cross the format's one field both ways: a
rejection is Adobe's `-1` and stars are stars, and stars arriving on a
rejected frame lift the rejection, since the file said it was worth a
number. An unrated file says nothing and clears nothing, on the rule the
`.drsc` merge keeps. `versions.label` finally has a reader and a writer,
with the code table moved out of the query so the two cannot drift.

core/dr-sync/src/scan.rs

@@ -37,6 +37,17 @@ pub struct ScanProgress {
 /// on the whole edit format to recognise four characters in a filename.
 pub const SIDECAR_EXTENSION: &str = "drsc";
 
+/// TRACES: FR-CAT-13
+/// Extension of a standard XMP sidecar — Lightroom's `IMG_0001.xmp`,
+/// darktable's `IMG_0001.CR3.xmp`, and every other editor's.
+///
+/// Collected alongside DarkRoom's own for the same reason and at the same
+/// cost: it is in the listing already, and it is the file the ratings and
+/// keywords of a library edited elsewhere are in. Which images a given
+/// `.xmp` describes is the catalog's question, since the two naming
+/// conventions resolve differently and only the catalog knows the images.
+pub const XMP_EXTENSION: &str = "xmp";
+
 /// The result of a scan.
 #[derive(Debug, Clone, Default)]
 pub struct ScanResult {
@@ -275,15 +286,18 @@ where
     Ok(result)
 }
 
-/// Whether a filename is a DarkRoom sidecar.
+/// Whether a filename is a sidecar — DarkRoom's own, or a standard XMP one.
 ///
 /// Case-insensitive on the extension alone. A server that upper-cased the
 /// suffix — or a file copied through a filesystem that did — still describes a
 /// photograph, and failing to recognise it would silently lose the edit rather
 /// than fail visibly.
 fn is_sidecar(name: &str) -> bool {
-    name.rsplit_once('.')
-        .is_some_and(|(stem, ext)| !stem.is_empty() && ext.eq_ignore_ascii_case(SIDECAR_EXTENSION))
+    name.rsplit_once('.').is_some_and(|(stem, ext)| {
+        !stem.is_empty()
+            && (ext.eq_ignore_ascii_case(SIDECAR_EXTENSION)
+                || ext.eq_ignore_ascii_case(XMP_EXTENSION))
+    })
 }
 
 /// Whether pruning is worth attempting against this backend.
@@ -927,6 +941,10 @@ mod tests {
                 // Upper-cased by a filesystem somewhere along the way; still a
                 // sidecar, and losing it would lose the edit silently.
                 file("Photos/2025/b.DRSC"),
+                // TRACES: FR-CAT-13
+                // And the standard kind, in both of its spellings.
+                file("Photos/2025/a.xmp"),
+                file("Photos/2025/b.jpg.xmp"),
             ],
         );
         let before = *b.lists.borrow();
@@ -946,14 +964,22 @@ mod tests {
                 .iter()
                 .map(|e| e.path.as_str().to_string())
                 .collect::<Vec<_>>(),
-            vec!["Photos/2025/a.drsc", "Photos/2025/b.DRSC"]
+            vec![
+                "Photos/2025/a.drsc",
+                "Photos/2025/a.xmp",
+                "Photos/2025/b.DRSC",
+                "Photos/2025/b.jpg.xmp",
+            ]
         );
         assert_eq!(*b.lists.borrow() - before, 3, "no extra requests");
 
         // And they are not photographs: the count the user is shown must not
         // double because a library has been edited.
         assert_eq!(r.progress.images_found, 3);
-        assert!(r.images.iter().all(|e| !e.path.name().contains("drsc")));
+        assert!(r
+            .images
+            .iter()
+            .all(|e| !e.path.name().contains("drsc") && !e.path.name().contains("xmp")));
     }
 
     /// A file whose *name* is only an extension is not a sidecar for anything.