Cut panorama overlaps along seams instead of averaging them

The merge weighted every overlap pixel by its distance from each frame's
edge, a 200 px linear cross-fade. Anything the frames disagreed on —
parallax in the near foreground, grass in the wind, a walker — came out
twice at half strength: a soft double edge at 1:1.

dr_pano::seam picks, per output texel at proxy resolution, which frame a
pixel comes from. Where a new frame overlaps the composite the cost is the
gain-corrected difference plus local detail plus nearness to either
frame's edge, taken as the worst over a small window, and the cut is a
dynamic-programming path across the overlap. merge.wgsl weights each frame
by its tent-filtered share of that map, a 64 px blend that follows the
seam, with the edge feather kept as the fallback. The page's preview uses
the same map, and examples/merge.rs takes --feather-only for comparison.

ui/dr-ui/examples/merge.rs

@@ -6,6 +6,9 @@
 //! cargo run -p dr-ui --example merge --release -- --leave-out 3 fixtures/pano/2025-08-05/*.CR2
 //! ```
 //!
+//! `--feather-only` merges without seams — every overlap the feathered
+//! average — for comparing the two on the same set.
+//!
 //! `--leave-out N` unticks frame `N` once the first alignment is in, as the
 //! page's box would, so the job aligns again without it.
 //!
@@ -25,6 +28,7 @@ fn main() {
     let mut fill = false;
     let mut wait_engines = false;
     let mut leave_out: Option<usize> = None;
+    let mut seams = true;
     loop {
         match args.first().map(String::as_str) {
             Some("--out") => {
@@ -39,6 +43,10 @@ fn main() {
                 args.remove(0);
                 leave_out = args.remove(0).parse().ok();
             }
+            Some("--feather-only") => {
+                args.remove(0);
+                seams = false;
+            }
             Some("--wait-engines") => {
                 args.remove(0);
                 wait_engines = true;
@@ -101,6 +109,7 @@ fn main() {
         dr_ui::merge::MergeRequest::new(frames, dr_ui::merge::MergeDestination::Local(dir));
     // The filler from the shared models directory, as the app finds it.
     request.inpaint_model = dr_ui::merge::inpaint_model_path();
+    request.seams = seams;
     let (tx, rx) = std::sync::mpsc::channel();
     let (decide, decision) = std::sync::mpsc::channel();
     let cancel = dr_ui::merge::Cancel::default();