Open a photograph with the edit it already carries, when DarkRoom has none

The library's DNGs carry the photographer's earlier develop settings
in their embedded XMP — the house style their photographs were made
with. A photograph opened with no edit of DarkRoom's now starts from
that earlier edit, translated (HSL bands, highlights, blacks and the
rest), as one undoable step named "Earlier Edit"; from there it is an
ordinary edit, saved with the photograph. Export does the same, so a
photograph never opened exports as opening it would show.

Only on positive evidence that there is no DarkRoom edit: a local file
with no sidecar beside it, or a server that answered "no such file"
with nothing in the cache. The stored-edit fetch now says which
(FetchedSidecar::absent). Offline, unreachable or unreadable never
counts — the earlier edit would otherwise be saved over a real edit
that merely failed to arrive.

ui/dr-ui/src/develop/session.rs

@@ -76,6 +76,11 @@ pub struct DevelopSession {
     /// the info panel offers to save for every photograph from that camera
     /// (D20). `None` once taken up.
     pub(super) profile_offer: Option<Arc<dr_decode::dcp::Dcp>>,
+    /// TRACES: FR-DEV-6
+    /// The edit the file already carries from the photographer's earlier work, translated, waiting for the
+    /// word that this photograph has no edit of DarkRoom's own
+    /// ([`Self::adopt_earlier_edit`]).
+    pub(super) earlier_edit: Option<Preset>,
     /// Kept so the session can build GPU resources after construction.
     ///
     /// The distance fields behind a subject mask are made when a layer is
@@ -439,6 +444,7 @@ impl DevelopSession {
             // and not found" — `lens_summary` distinguishes them.
             lens_profile_found: false,
             profile_offer: None,
+            earlier_edit: None,
             ctx: ctx.clone(),
             graph,
             history,
@@ -610,6 +616,33 @@ impl DevelopSession {
             .map(Arc::new);
     }
 
+    /// TRACES: FR-DEV-6
+    /// Remember the earlier edit stored in the file this session opened.
+    pub fn set_earlier_edit(&mut self, edit: Option<Preset>) {
+        self.earlier_edit = edit;
+    }
+
+    /// TRACES: FR-DEV-6
+    /// Apply the file's earlier edit, as the photograph's starting point, so it
+    /// keeps the style it was given before it came to DarkRoom.
+    ///
+    /// Only for a photograph DarkRoom has no edit of: the caller says so, and
+    /// must know it rather than guess — a stored edit that could not be
+    /// fetched is not an absent one, and the earlier edit would then be
+    /// saved over it on the way out. One undoable step, so undo shows the
+    /// photograph without it; from there it is an ordinary edit, saved with
+    /// the photograph. Taken, so a second call does nothing.
+    pub fn adopt_earlier_edit(&mut self) -> bool {
+        let Some(edit) = self.earlier_edit.take() else {
+            return false;
+        };
+        let rebake = edit.apply(&mut self.graph, Scope::adjustments());
+        self.pay_film_debt(&rebake);
+        self.history
+            .record(&self.graph, Edit::Action(labels::step::EARLIER_EDIT));
+        true
+    }
+
     /// TRACES: FR-DEV-3e
     /// What to tell the photographer about the camera profile (D20).
     ///
@@ -888,6 +921,34 @@ mod tests {
         assert_eq!(jpeg.profile_summary(), "");
     }
 
+    #[test]
+    fn a_earlier_edit_is_adopted_once_and_can_be_undone() {
+        // TRACES: FR-DEV-6
+        let Some(ctx) = headless() else { return };
+        let mut s = DevelopSession::open(&ctx, &raw_with(None), dr_types::Orientation::NORMAL)
+            .expect("session");
+        assert!(!s.adopt_earlier_edit(), "nothing to adopt yet");
+
+        let earlier = dr_preset_xmp::read_xmp(
+            r#"<x:xmpmeta xmlns:x="adobe:ns:meta/"><rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"><rdf:Description rdf:about="" xmlns:crs="http://ns.adobe.com/camera-raw-settings/1.0/" crs:SaturationAdjustmentBlue="+58" crs:Highlights2012="-40"/></rdf:RDF></x:xmpmeta>"#,
+        )
+        .expect("an edit")
+        .preset;
+        s.set_earlier_edit(Some(earlier));
+        assert!(s.adopt_earlier_edit());
+        let blue = || {
+            s.graph.param(
+                dr_pipeline::ops::colour_mixer::ID,
+                dr_pipeline::ParamId("blue_sat"),
+            )
+        };
+        assert_eq!(blue(), Some(58.0));
+        assert!(!s.adopt_earlier_edit(), "taken: a second call does nothing");
+        assert!(!s.graph.is_neutral());
+        s.undo();
+        assert!(s.graph.is_neutral(), "undo shows the photograph without it");
+    }
+
     #[test]
     fn only_a_copyable_profile_is_offered() {
         // TRACES: FR-DEV-3e