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/semantic.rs

@@ -56,13 +56,14 @@ const DETECTION_STRIDE: usize = 6 + PROTOTYPES;
 const PROTO_STRIDE: usize = 4;
 
 /// How the image is presented to a fixed-shape graph.
-#[derive(Debug, Clone, Copy, PartialEq)]
+#[derive(Debug, Clone, Copy, PartialEq, Default)]
 pub enum Tiling {
     /// One inference over the whole frame, letterboxed into the square input.
     ///
     /// The default, and the right default: a photographic subject is usually
     /// *large* in frame, which is the case whole-image inference handles best
     /// and the case tiling helps least.
+    #[default]
     Whole,
     /// Cover the frame with overlapping fixed-size windows.
     ///
@@ -77,12 +78,6 @@ pub enum Tiling {
     Grid { overlap: f32 },
 }
 
-impl Default for Tiling {
-    fn default() -> Self {
-        Self::Whole
-    }
-}
-
 /// How the semantic pass is tuned.
 #[derive(Debug, Clone, Copy, PartialEq)]
 pub struct SemanticOptions {
@@ -515,7 +510,7 @@ impl Letterbox {
     }
 
     /// Input-space box back to source pixels.
-    fn to_source(&self, x0: f32, y0: f32, x1: f32, y1: f32, w: &Window) -> (f32, f32, f32, f32) {
+    fn to_source(self, x0: f32, y0: f32, x1: f32, y1: f32, w: &Window) -> (f32, f32, f32, f32) {
         (
             (x0 - self.pad_x) / self.scale + w.x,
             (y0 - self.pad_y) / self.scale + w.y,
@@ -525,7 +520,7 @@ impl Letterbox {
     }
 
     /// Source pixel to prototype-grid coordinates.
-    fn to_proto(&self, sx: f32, sy: f32, w: &Window) -> (f32, f32) {
+    fn to_proto(self, sx: f32, sy: f32, w: &Window) -> (f32, f32) {
         (
             ((sx - w.x) * self.scale + self.pad_x) / PROTO_STRIDE as f32,
             ((sy - w.y) * self.scale + self.pad_y) / PROTO_STRIDE as f32,