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/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())));
     };