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.
     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.
  -  and  take  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,
-    }
-}

ui/dr-ui/src/export.rs

@@ -672,23 +672,26 @@ fn carried_metadata(meta: &dr_decode::Metadata) -> dr_export::SourceMetadata {
     }
 }
 
+/// One rendered photograph on its way to a file: the name it will be written
+/// under, the name it came from, the pixels, and whatever metadata travelled
+/// with them.
+type RenderedItem = (
+    String,
+    String,
+    dr_export::Frame,
+    Option<dr_export::SourceMetadata>,
+);
+
 /// Fetch a photograph, apply its stored edit, and render it at full size.
+///
+/// `None` means the work was cancelled, which is not a failure and has no
+/// error to report — hence the `Option` outside the `Result`.
 fn render_from_library(
     request: &BatchRequest,
     path: &str,
     cache: Option<crate::library::CacheContext>,
     cancel: &Cancel,
-) -> Option<
-    Result<
-        (
-            String,
-            String,
-            dr_export::Frame,
-            Option<dr_export::SourceMetadata>,
-        ),
-        ItemError,
-    >,
-> {
+) -> Option<Result<RenderedItem, ItemError>> {
     let Some((creds, user_id)) = request.creds.clone() else {
         return Some(Err(ItemError::Fetch("no library is open".into())));
     };