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>

core/dr-decode/src/locate.rs

@@ -1201,7 +1201,8 @@ mod tests {
         let heap = GPS_IFD + 2 + 12 + 4;
         let mut extra = Vec::new();
         extra.extend_from_slice(&1u16.to_le_bytes());
-        for (tag, kind, count, value) in [(gps_tag::LATITUDE, 5u16, 3u32, heap)] {
+        {
+            let (tag, kind, count, value) = (gps_tag::LATITUDE, 5u16, 3u32, heap);
             extra.extend_from_slice(&tag.to_le_bytes());
             extra.extend_from_slice(&kind.to_le_bytes());
             extra.extend_from_slice(&count.to_le_bytes());