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-gpu/tests/capture_sharpen.rs

@@ -296,7 +296,11 @@ fn the_threshold_leaves_shallow_modulation_where_it_found_it() {
     // under the 12.5% at which the gate below starts letting detail through.
     let ripple: Vec<u8> = (0..SIZE * SIZE)
         .flat_map(|i| {
-            let v = if (i % SIZE) % 2 == 0 { 128u8 } else { 132 };
+            let v = if (i % SIZE).is_multiple_of(2) {
+                128u8
+            } else {
+                132
+            };
             [v, v, v, 255]
         })
         .collect();