Keep colour labels in the sidecar so they survive and travel

A rating and a flag are written to DarkRoom's sidecar as well as the
catalog, because the catalog is a disposable index and the sidecar is
how a judgement reaches the photographer's other devices. A label had no
place there, so once labels could be set, one would have lived only in
the catalog of the device it was set on and gone with it.

The sidecar version now carries `label` (0 none, 1-5 as the catalog
codes it), written only when set. It merges under the rating's rule, so a
device that never labelled a frame cannot clear another device's label,
and a code this build does not know reads as none rather than as some
other colour. A judgement write carries the catalog's label with the
stars, and the scan takes a sidecar's label into the catalog when it has
one. An older build keeps the line as an unknown key and writes it back.

ui/dr-ui/src/library/sidecar.rs

@@ -34,8 +34,8 @@ pub struct SidecarWrite {
 /// amendment rather than a replacement.
 #[derive(Debug, Clone)]
 pub enum Amendment {
-    /// A star rating and a pick/reject flag — the cull.
-    Judgement { rating: u8, flag: u8 },
+    /// A star rating, a pick/reject flag and a colour label — the cull.
+    Judgement { rating: u8, flag: u8, label: u8 },
     /// TRACES: FR-DEV-6
     /// Copied develop settings, applied within `scope`.
     ///
@@ -302,9 +302,14 @@ pub(super) fn amend(base: dr_pipeline::Sidecar, w: &SidecarWrite) -> dr_pipeline
     // paste must spare, the unknown keys of an operation this build lacks —
     // survives because it was read from the file and is written back.
     match &w.amendment {
-        Amendment::Judgement { rating, flag } => {
+        Amendment::Judgement {
+            rating,
+            flag,
+            label,
+        } => {
             version.rating = *rating;
             version.flag = *flag;
+            version.label = *label;
         }
         Amendment::Settings {
             preset,
@@ -624,7 +629,11 @@ mod tests {
         SidecarWrite {
             image_path: "PhotosRaw/incoming/a.CR2".to_string(),
             version_uuid: uuid.to_string(),
-            amendment: Amendment::Judgement { rating, flag: 0 },
+            amendment: Amendment::Judgement {
+                rating,
+                flag: 0,
+                label: 0,
+            },
         }
     }