The merge job and its page: a selection to a panorama DNG, confirmed first

dr_ui::merge is the orchestration with no interface in it: decode each
frame to sensor data and build its graph as a session would (orientation,
lens profile); render each through the camera-space tap at proxy size and
detect keypoints there, so the alignment is measured in the undistorted
frame the tiles are rendered in; align; solve one gain per frame from the
proxies' overlaps; draw the aligned set in colour for the page; then wait.
Nothing is written until a Decision arrives (FR-MRG-1). The merge writes
a linear DNG through the outbox with a destination record, so the drain
puts it beside its sources on a folder library and a server alike, and
the library rescans (FR-MRG-3).

merge.slint is the page, on the import page's model: the alignment
table with a failed frame named on its row and the button held off
(FR-MRG-5), the preview, the projection choice, Stop and Back. A
"Merge to panorama" button joins the grid's selection bar at two frames.

Headless, the example produces the fixture's 22 993 x 5 980 DNG in 45 s
on the reference desktop, exposures balanced across the stop of drift.

ui/dr-ui/src/develop.rs

@@ -4578,7 +4578,7 @@ impl DevelopSession {
     /// lens's distortion and not its vignetting, or covers only part of a
     /// zoom's range, and a partial profile is worth applying — discarding it
     /// because one field is missing would turn a good correction into none.
-    fn profile_for(meta: &dr_decode::Metadata) -> Option<dr_pipeline::LensProfile> {
+    pub(crate) fn profile_for(meta: &dr_decode::Metadata) -> Option<dr_pipeline::LensProfile> {
         // All three are needed to ask the question at all. A lens name alone
         // does not identify a correction: distortion is interpolated across a
         // zoom's focal range and vignetting depends strongly on aperture — a