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.
 

docs/dev/requirements.md

@@ -2437,7 +2437,7 @@ Rationale, evidence, and the eliminated alternatives are recorded in
 | D12 | Scope versus pace | **DECIDED 2026-09-19** — settled by events; full scope stands, no v1 date |
 | D18 | Derived images | **DECIDED 2026-09-19** — a merge writes a new source file; no multi-source Version |
 | D19 | Scene-referred pipeline | **DECIDED 2026-09-27** — edits on unbounded scene-linear colour; one view transform, last; per-body base curves retired |
-| D21 | DNG reference tone for raws | **PARTLY DECIDED 2026-10-03** — the view transform gains the DNG reference curve (profile's, else ACR3 default, via RGBTone in ProPhoto) as a choice; which curve is the default is open, pending measurement against Lightroom exports |
+| D21 | DNG reference tone for raws | **DECIDED 2026-10-03** — the view transform's DNG reference curve (profile's, else ACR3 default, via RGBTone in ProPhoto) is the default for every raw, at contrast 1.5 (a ×1.07 power about grey); measured against Lightroom exports of photographs with neutral look settings; the sigmoid stays a choice |
 | D20 | DCP camera profiles | **DECIDED 2026-10-02** — HueSatMap and LookTable as a scene operation after exposure; embedded profile first, then a matched `.dcp`; tone curve not applied; none shipped |
 
 ### D11 — product positioning
@@ -2773,9 +2773,30 @@ matrix does.
 *What it costs.* Every DNG with an embedded profile renders differently; previews refresh only when
 rendered again; tablet and desktop release together. The profiles directory does not sync yet.
 
-### D21 — DNG reference tone for raws · **PARTLY DECIDED 2026-10-03**
+### D21 — DNG reference tone for raws · **DECIDED 2026-10-03**
 
-*Amended the same day:* the default is **not** decided. The measurement below was against
+*Decided by measurement, later the same day.* The library's photo gallery holds Lightroom 6
+exports of raws that are in the library, each carrying its Camera Raw settings. Clustered by
+those settings, 663 exports had none of the house look (Linear curve, no HSL, no parametric
+curve, no split toning); 60 of them with their raws, two thirds fitted and one third held out,
+rendered by DarkRoom against Lightroom's JPEG (MSE, sRGB 8-bit):
+
+| Rendering | Held-out MSE |
+|---|---|
+| 0.20.0's sigmoid at its defaults | ~1200 — about 0.7 EV darker, and flatter |
+| Sigmoid, exposure, contrast and white fitted | ~150 (contrast 1.73, +0.73 EV) |
+| Camera Raw's curve after baseline exposure, at contrast 1.4 | 224 |
+| Camera Raw's curve, contrast 1.5 | ~150 |
+| Camera Raw's curve, exposure, contrast and white fitted | 143 |
+
+So Camera Raw's curve is the default for every raw, and the default contrast is 1.5 — under that
+curve a power of 1.5/1.4 about grey (`REFERENCE_CONTRAST` is where the curve is untouched). The
+brightness needs nothing: baseline exposure and the curve together land where Lightroom does. The
+profile's look strength, vibrance and saturation bought nothing measurable on those exports. The
+user chose to change every photograph rather than keep edited ones on the old rendering. The
+fitting tools live outside the repository (`darkroom-lrfit`).
+
+*Amended earlier the same day:* the default was **not** decided. The measurement below was against
 Lightroom previews of photographs carrying the user's Lightroom edits — a house look of HSL
 saturation (Blue +58, Aqua +50, …), Highlights −40 and Blacks −20 in every DNG's XMP — so it said
 nothing about Camera Raw's base rendering. Under Camera Raw's curve `_MG_9080` renders brighter