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/lib.rs

@@ -334,6 +334,10 @@ pub(crate) fn open_session(
     if dr_decode::probe(bytes) != Some(dr_types::Format::Jpeg) {
         session.set_embedded_profile(dr_decode::dcp::embedded_in(bytes));
     }
+    // TRACES: FR-DEV-6
+    // And the earlier edit stored in it, applied by the caller only once it
+    // knows DarkRoom has no edit of this photograph.
+    session.set_earlier_edit(dr_preset_xmp::read_embedded(bytes).map(|i| i.preset));
     Ok(session)
 }
 
@@ -820,7 +824,7 @@ pub(crate) fn refresh_export_label(window: &AppWindow) {
 /// whose session this sidecar must not be applied to.
 fn apply_when_ready(
     window: &AppWindow,
-    rx: Rc<std::sync::mpsc::Receiver<Option<dr_pipeline::Sidecar>>>,
+    rx: Rc<std::sync::mpsc::Receiver<library::FetchedSidecar>>,
     session: &Rc<RefCell<Option<DevelopSession>>>,
     rows: &Rc<slint::VecModel<ParamRow>>,
     redraw: &Rc<dyn Fn(&AppWindow)>,
@@ -830,9 +834,7 @@ fn apply_when_ready(
 
     // Already here — the common case.
     if let Ok(got) = rx.try_recv() {
-        if let Some(sidecar) = got {
-            presets::apply_stored_edit(window, &sidecar, session, rows);
-        }
+        apply_fetched(window, got, session, rows);
         redraw(window);
         return;
     }
@@ -867,10 +869,7 @@ fn apply_when_ready(
                 return;
             };
             held.stop();
-            let applied = match got {
-                Some(sidecar) => presets::apply_stored_edit(&w, &sidecar, &session, &rows),
-                None => false,
-            };
+            let applied = apply_fetched(&w, got, &session, &rows);
             if applied || !drawn.get() {
                 redraw(&w);
             }
@@ -878,6 +877,42 @@ fn apply_when_ready(
     );
 }
 
+/// TRACES: FR-CAT-8 | FR-DEV-6
+/// Apply what the stored-edit fetch answered: the edit, or — only where the
+/// server said there is none — the photograph's earlier edit. An answer
+/// that could not be had (offline, unreachable, unreadable) applies nothing,
+/// so a real edit that failed to arrive is never saved over.
+fn apply_fetched(
+    window: &AppWindow,
+    got: library::FetchedSidecar,
+    session: &Rc<RefCell<Option<DevelopSession>>>,
+    rows: &Rc<slint::VecModel<ParamRow>>,
+) -> bool {
+    match got.sidecar {
+        Some(sidecar) => presets::apply_stored_edit(window, &sidecar, session, rows),
+        None if got.absent => adopt_earlier_edit(window, session, rows),
+        None => false,
+    }
+}
+
+/// TRACES: FR-DEV-6
+/// Start the open photograph from its earlier edit, if it has one.
+fn adopt_earlier_edit(
+    window: &AppWindow,
+    session: &Rc<RefCell<Option<DevelopSession>>>,
+    rows: &Rc<slint::VecModel<ParamRow>>,
+) -> bool {
+    let adopted = session
+        .borrow_mut()
+        .as_mut()
+        .is_some_and(DevelopSession::adopt_earlier_edit);
+    if adopted {
+        log::info!("started from the photograph's earlier edit");
+        sync_rows(window, rows, session);
+    }
+    adopted
+}
+
 /// TRACES: FR-DEV-3f
 /// Push the film choice out to the panel.
 ///
@@ -2623,6 +2658,11 @@ fn build_show(
                             *open_image.borrow_mut() = presets::Stored::Local(path.to_path_buf());
                             if let Some(sidecar) = presets::load_local(path) {
                                 presets::apply_stored_edit(window, &sidecar, &session, &rows);
+                            } else {
+                                // TRACES: FR-DEV-6
+                                // No edit of DarkRoom's beside the file: start
+                                // from the earlier edit stored in it.
+                                adopt_earlier_edit(window, &session, &rows);
                             }
 
                             // TRACES: FR-UI-4