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/ops/view_transform.rs

@@ -34,10 +34,13 @@ pub const ID: OpId = OpId("view_transform");
 pub const CONTRAST: ParamId = ParamId("contrast");
 pub const WHITE: ParamId = ParamId("white");
 /// TRACES: FR-DEV-3j
-/// Which curve renders: D19's sigmoid (index 0, the default) or Camera
-/// Raw's (D21). Which of the two should be the default is open — the
-/// measurement work decides it — so the default is what 0.20.0
-/// rendered.
+/// Which curve renders: Camera Raw's (D21, the default) or D19's sigmoid.
+///
+/// Camera Raw's because it is the one that matches what the photographs were
+/// first developed with: on 60 Lightroom exports with neutral settings it
+/// renders their raws within MSE ~150 of Lightroom's own JPEGs at the default
+/// contrast, where 0.20.0's sigmoid was ~1200 (darker by about 0.7 EV and
+/// flatter). The sigmoid keeps index 0 because sidecars record the index.
 pub const CURVE: ParamId = ParamId("curve");
 
 /// [`CURVE`]'s values, in the order of its variants.
@@ -93,7 +96,8 @@ static DESCRIPTOR: LazyLock<Arc<OpDescriptor>> = LazyLock::new(|| {
                     LocalizedKey("param.view_transform.curve.sigmoid"),
                     LocalizedKey("param.view_transform.curve.camera_raw"),
                 ],
-            ),
+            )
+            .with_default(CAMERA_RAW),
         ],
     })
 });
@@ -110,7 +114,7 @@ impl Default for ViewTransform {
         Self {
             contrast: DEFAULT_CONTRAST,
             white: DEFAULT_WHITE,
-            curve: SIGMOID,
+            curve: CAMERA_RAW,
         }
     }
 }
@@ -150,7 +154,7 @@ impl Operation for ViewTransform {
     }
 
     fn is_active(&self) -> bool {
-        self.contrast != DEFAULT_CONTRAST || self.white != DEFAULT_WHITE || self.curve != SIGMOID
+        self.contrast != DEFAULT_CONTRAST || self.white != DEFAULT_WHITE || self.curve != CAMERA_RAW
     }
 
     fn stage(&self) -> Stage {