Reconcile the three branches

Two faults textual merge could not see. Both agents added a `mod tests` to
masks_ui.rs — the module boundary was an artefact of them being written
apart, and the tests do not overlap, so they fold into one. And `segment`
lost its context argument when the work moved to a worker, which a test
written on another branch still passed.

ui/dr-ui/src/develop.rs

@@ -2464,7 +2464,7 @@ mod tests {
 
         let before = session.mask_raster_size();
         if session
-            .segment(&ctx, &crate::segmentation::Options::default())
+            .segment(&crate::segmentation::Options::default())
             .is_err()
         {
             eprintln!("no model; skipping");