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-pipeline/src/operation.rs

@@ -707,7 +707,7 @@ pub fn compose_full(
         "    // The camera profile's base curve and the conversion out of camera\n    // space are both absent: an operation declaring `Operation::renders`\n    // has done both, and doing them again would render the picture twice.\n"
             .to_string()
     } else {
-                    "    // ==== camera profile: the base curve (FR-DEV-3e) ====
+        "    // ==== camera profile: the base curve (FR-DEV-3e) ====
     //
     // Marked with `====` and not the `----` an operation block carries: this
     // is not one, and the difference is what several tests count on to tell