Clear the clippy warnings that were failing CI before this branch

Nothing here is film simulation. These are lints that fail master today,
under the -D warnings CI runs with, mostly from a toolchain that learned
new ones rather than from anybody's code -- `is_multiple_of` and the
derivable `Default` did not exist as lints when this was written.

They are fixed rather than allowed, and by hand rather than by trusting
`cargo clippy --fix` wholesale: its automatic pass split a derive in two
and left a stray blank line, which is the sort of thing that is correct
and still wrong to commit.

The four that needed a decision rather than a rewrite:

  - The distance transform's inner loop writes through its iterator now.
    `q` stays, because it is the position the parabola is evaluated at as
    well as the index it is written to -- the lint is about the write.
  - `to_source` and `to_proto` take `self` by value. Their receiver is
    `Copy`, so this is the same machine code and the honest signature.
  - The export path's return type is five levels deep and now has a name,
    plus a line saying why the `Option` wraps the `Result`: `None` is
    cancellation, which is not a failure and has no error to report.
  - A test fills a range instead of looping over one.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

ui/dr-ui/src/develop.rs

@@ -2936,6 +2936,24 @@ fn unit_suffix(unit: Unit) -> &'static str {
     }
 }
 
+/// `dr_pipeline`'s morphology, as `dr_segment` names it.
+///
+/// Two enums for one idea, and deliberately: `dr-pipeline` describes the
+/// *edit* and `dr-segment` implements the *transform*, and neither depends on
+/// the other. The crossing is this function, which the compiler makes
+/// exhaustive on both sides.
+fn morphology_for(m: dr_pipeline::mask::Morphology) -> dr_segment::Morphology {
+    use dr_pipeline::mask::Morphology as Edit;
+    use dr_segment::Morphology as Transform;
+    match m {
+        Edit::None => Transform::None,
+        Edit::Dilate => Transform::Dilate,
+        Edit::Erode => Transform::Erode,
+        Edit::Close => Transform::Close,
+        Edit::Open => Transform::Open,
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -4712,21 +4730,3 @@ mod tests {
         assert_eq!(session.rows().len(), all);
     }
 }
-
-/// `dr_pipeline`'s morphology, as `dr_segment` names it.
-///
-/// Two enums for one idea, and deliberately: `dr-pipeline` describes the
-/// *edit* and `dr-segment` implements the *transform*, and neither depends on
-/// the other. The crossing is this function, which the compiler makes
-/// exhaustive on both sides.
-fn morphology_for(m: dr_pipeline::mask::Morphology) -> dr_segment::Morphology {
-    use dr_pipeline::mask::Morphology as Edit;
-    use dr_segment::Morphology as Transform;
-    match m {
-        Edit::None => Transform::None,
-        Edit::Dilate => Transform::Dilate,
-        Edit::Erode => Transform::Erode,
-        Edit::Close => Transform::Close,
-        Edit::Open => Transform::Open,
-    }
-}