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>

core/dr-segment/src/distance.rs

@@ -183,12 +183,15 @@ fn transform(f: &[f32], out: &mut [f32], v: &mut [usize], z: &mut [f32]) {
     }
 
     k = 0;
-    for q in 0..n {
+    // `q` is both the position and the value the parabola is evaluated at, so
+    // it stays; what changes is that the write goes through the iterator
+    // rather than re-indexing `out` on every step.
+    for (q, slot) in out.iter_mut().enumerate().take(n) {
         while z[k + 1] < q as f32 {
             k += 1;
         }
         let p = v[k];
-        out[q] = (q as f32 - p as f32).powi(2) + f[p];
+        *slot = (q as f32 - p as f32).powi(2) + f[p];
     }
 }
 
@@ -414,13 +417,13 @@ mod tests {
 
     #[test]
     fn an_empty_mask_is_everywhere_outside() {
-        let d = signed_distance(&vec![0u8; 16], 4, 4, 128);
+        let d = signed_distance(&[0u8; 16], 4, 4, 128);
         assert!(d.iter().all(|&v| v < 0.0), "no pixel can be inside");
     }
 
     #[test]
     fn a_full_mask_is_everywhere_inside() {
-        let d = signed_distance(&vec![255u8; 16], 4, 4, 128);
+        let d = signed_distance(&[255u8; 16], 4, 4, 128);
         assert!(d.iter().all(|&v| v > 0.0), "no pixel can be outside");
     }
 
@@ -548,9 +551,7 @@ mod tests {
         // M5: masks reach the sidecar as indices, so the field they are shaped
         // by has to be reproducible.
         let mut m = vec![0u8; 64];
-        for i in 20..30 {
-            m[i] = 255;
-        }
+        m[20..30].fill(255);
         let a = signed_distance(&m, 8, 8, 128);
         let b = signed_distance(&m, 8, 8, 128);
         assert_eq!(a, b);