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/src/adjust.rs

@@ -165,7 +165,12 @@ fn film_key(t: &dr_pipeline::ops::FilmTables) -> u64 {
             mix(v.to_bits());
         }
     }
-    for v in [t.curve_log_min, t.curve_log_max, t.density_max, t.lut_size as f32] {
+    for v in [
+        t.curve_log_min,
+        t.curve_log_max,
+        t.density_max,
+        t.lut_size as f32,
+    ] {
         mix(v.to_bits());
     }
     for e in t.lut.iter().step_by(8).chain(t.curves.iter().step_by(8)) {
@@ -228,7 +233,11 @@ impl AdjustPass {
         let lut = self.upload_film(
             "adjust-film-lut",
             wgpu::TextureDimension::D3,
-            wgpu::Extent3d { width: n, height: n, depth_or_array_layers: n },
+            wgpu::Extent3d {
+                width: n,
+                height: n,
+                depth_or_array_layers: n,
+            },
             &to_rgba(&t.lut),
         );
         self.film = Some(FilmTextures { curves, lut, key });
@@ -282,7 +291,9 @@ impl AdjustPass {
 
     /// The curve texture to bind: the loaded stock's, or the placeholder.
     fn film_curves_view(&self) -> &wgpu::TextureView {
-        self.film.as_ref().map_or(&self.empty_film_curves, |f| &f.curves)
+        self.film
+            .as_ref()
+            .map_or(&self.empty_film_curves, |f| &f.curves)
     }
 
     /// The density lookup to bind: the loaded stock's, or the placeholder.
@@ -352,7 +363,11 @@ impl AdjustPass {
             .device
             .create_texture(&wgpu::TextureDescriptor {
                 label: Some("adjust-empty-film-curves"),
-                size: wgpu::Extent3d { width: 1, height: 1, depth_or_array_layers: 1 },
+                size: wgpu::Extent3d {
+                    width: 1,
+                    height: 1,
+                    depth_or_array_layers: 1,
+                },
                 mip_level_count: 1,
                 sample_count: 1,
                 dimension: wgpu::TextureDimension::D2,
@@ -365,7 +380,11 @@ impl AdjustPass {
             .device
             .create_texture(&wgpu::TextureDescriptor {
                 label: Some("adjust-empty-film-lut"),
-                size: wgpu::Extent3d { width: 1, height: 1, depth_or_array_layers: 1 },
+                size: wgpu::Extent3d {
+                    width: 1,
+                    height: 1,
+                    depth_or_array_layers: 1,
+                },
                 mip_level_count: 1,
                 sample_count: 1,
                 dimension: wgpu::TextureDimension::D3,