Render raws through the DNG reference curve by default, at contrast 1.5

Decides D21 by measurement. The photo gallery holds Lightroom 6 exports of
raws in the library, each carrying its Camera Raw settings; clustered by
those settings, 663 had no look applied. On 60 of them with their raws,
a third held out, the held-out MSE against Lightroom's JPEG was about 1200
for 0.20.0's sigmoid (0.7 EV darker and flatter), 224 for the DNG reference curve
after baseline exposure, and about 140 once its input is bent by 1.5/1.4
about grey.

So the curve choice defaults to the DNG reference, keeping its index (sidecars
record it), and the default contrast is 1.5. Contrast under the DNG reference curve is
now a power relative to REFERENCE_CONTRAST (1.4), where the table is
untouched; the sigmoid at that contrast still matches the retired base
curve. JPEGs are unaffected: the view transform skips a rendered source.

core/dr-pipeline/src/descriptor.rs

@@ -464,6 +464,15 @@ impl ParamDescriptor {
         }
     }
 
+    /// The same choice with another variant as its default.
+    ///
+    /// For a choice whose variants were numbered before its default was
+    /// settled: a sidecar records the index, so reordering the variants to
+    /// put the default first would change what saved edits mean.
+    pub fn with_default(self, default: f32) -> Self {
+        Self { default, ..self }
+    }
+
     /// A 0…1 fraction — a proportion of something, rather than an amount.
     ///
     /// Its own constructor because the crop rect needs four of them and the