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.

docs/dev/camera-profiles.md

@@ -295,10 +295,13 @@ that rendering, chosen by a new parameter on `view_transform`:
 
 | `curve` | What it is | Default for |
 |---|---|---|
-| Sigmoid | D19's log-logistic curve | every raw, for now |
-| DNG reference | the profile's curve, else ACR3, via RGBTone in ProPhoto | — (D21: the default is open) |
+| Sigmoid | D19's log-logistic curve | — (a choice) |
+| DNG reference | the profile's curve, else ACR3, via RGBTone in ProPhoto | every raw (D21) |
 
-*Amended 2026-10-03:* the DNG reference was the default in the first draft. The measurement it rested on
+*Decided 2026-10-03:* the DNG reference is the default, at contrast 1.5 — measured against
+Lightroom exports of photographs with neutral look settings (D21 has the table).
+
+*Amended earlier on 2026-10-03:* the DNG reference was the default in the first draft. The measurement it rested on
 compared against Lightroom renders of *edited* photographs; see D21. The default is decided by
 measuring against Lightroom exports of unedited ones.