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/masks_ui.rs

@@ -843,11 +843,7 @@ mod tests {
             "a gradient nobody has selected is not being edited"
         );
     }
-}
 
-#[cfg(test)]
-mod tests {
-    use super::*;
 
     /// The ordinary case: the answer comes back to the photograph that asked.
     #[test]