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-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