atmosphere: say the NaN case out loud

!(t > 0.0) trips clippy's partial-ord lint under the pinned 1.94 toolchain,
which blocks the whole gate. t.is_nan() || t <= 0.0 is the same predicate
written so the NaN intent is legible, with a test pinning both ends.

src/ui/atmosphere.rs

@@ -224,7 +224,7 @@ impl Atmosphere {
     /// extrapolated colour: a transition is a journey between two rooms she
     /// chose, and there is nothing past either end of it.
     pub fn lerp(self, other: Atmosphere, t: f32) -> Atmosphere {
-        if self == other || !(t > 0.0) {
+        if self == other || t.is_nan() || t <= 0.0 {
             return self;
         }
         if t >= 1.0 {
@@ -243,6 +243,19 @@ impl Atmosphere {
 mod tests {
     use super::*;
 
+    #[test]
+    fn a_blend_outside_the_journey_resolves_to_an_endpoint() {
+        // `!(t > 0.0)` said this and tripped clippy's partial-ord lint, which
+        // blocks the CI gate. NaN and every t <= 0 must still land on `self`.
+        let a = Atmosphere::MintTea;
+        let b = Atmosphere::NeonGlow;
+        assert_eq!(a.lerp(b, f32::NAN), a, "NaN is not a place on the journey");
+        assert_eq!(a.lerp(b, 0.0), a);
+        assert_eq!(a.lerp(b, -1.0), a);
+        assert_eq!(a.lerp(b, 1.0), b);
+        assert_eq!(a.lerp(b, 2.0), b);
+    }
+
     #[test]
     fn a_blend_is_a_value_between_the_two_rooms() {
         let mid = Atmosphere::MintTea.lerp(Atmosphere::NeonGlow, 0.5);