Emit floats an f32 can hold, and drop the format! that formats nothing

CI runs cargo fmt --check and clippy -D warnings, and this branch had
never been through either. Both would have failed it.

The bulk was the generated colour tables: eight significant figures where
an f32 carries about 7.2, so the eighth is noise that rounds away at
compile time and clippy's excessive_precision says so 109 times over.
Fixed in the generator rather than only in the file, so it stays fixed --
and the file is trimmed in place rather than re-derived, because
regenerating it needs a colour-science stack that has nothing to do with
the defect.

The format! in the composer is mine too, from extracting the rendering
tail: the braces in it were escaped because the text used to live inside a
larger template, and once extracted the escapes are noise and the call
formats nothing.

Also here, and clearly not mine: an unused import and a shadowed binding
in dr-gpu, and an unused import in a test. They are pre-existing --
clippy has been failing on master before this branch existed, on lints
like is_multiple_of that arrived with a toolchain rather than with
anyone's code. Fixed because CI cannot go green around them, and called
out because a merge commit is a bad place to quietly edit someone else's
crate.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

core/dr-film/src/spectrum.rs

@@ -15,7 +15,7 @@ pub type Spectrum = [f32; SPECTRUM];
 /// happens before this is applied — see [`Viewing::to_srgb`].
 const XYZ_TO_SRGB: [[f32; 3]; 3] = [
     [3.2404542, -1.5371385, -0.4985314],
-    [-0.9692660, 1.8760108, 0.0415560],
+    [-0.969266, 1.8760108, 0.041556],
     [0.0556434, -0.2040259, 1.0572252],
 ];
 
@@ -81,8 +81,8 @@ pub fn blackbody(kelvin: f32) -> Spectrum {
     let mut out = [0.0f32; SPECTRUM];
     let mut total = 0.0f64;
     for (i, slot) in out.iter_mut().enumerate() {
-        let lambda = f64::from(crate::tables::LAMBDA_MIN + crate::tables::LAMBDA_STEP * i as f32)
-            * 1e-9;
+        let lambda =
+            f64::from(crate::tables::LAMBDA_MIN + crate::tables::LAMBDA_STEP * i as f32) * 1e-9;
         let radiance = (2.0 * H * C * C)
             / (lambda.powi(5) * ((H * C / (lambda * KB * f64::from(kelvin))).exp() - 1.0));
         *slot = radiance as f32;
@@ -208,7 +208,10 @@ mod tests {
         for r in 0..3 {
             for c in 0..3 {
                 let expected = if r == c { 1.0 } else { 0.0 };
-                assert!((m[r][c] - expected).abs() < 1e-5, "{m:?} is not the identity");
+                assert!(
+                    (m[r][c] - expected).abs() < 1e-5,
+                    "{m:?} is not the identity"
+                );
             }
         }
     }