Keep the sigmoid as the default curve; Camera Raw's tone is a choice

The Camera Raw default rested on comparing against Lightroom previews
of photographs that carry the user's Lightroom edits — HSL saturation
Blue +58, Aqua +50 and more, Highlights -40, Blacks -20, in every
DNG's XMP — so it measured the house look, not Camera Raw's base
rendering. Under the ACR3 curve _MG_9080 renders brighter than its
Lightroom preview (mean 0.39 against 0.31).

So the curve choice's first variant, the default, is the sigmoid again
and every raw renders as in 0.20.0 apart from baseline exposure. D21
and camera-profiles.md §12 now say the default is open, to be decided
by measuring against Lightroom exports of unedited photographs. Tests
that are about Camera Raw's tone choose it explicitly.

docs/dev/camera-profiles.md

@@ -295,8 +295,12 @@ that rendering, chosen by a new parameter on `view_transform`:
 
 | `curve` | What it is | Default for |
 |---|---|---|
-| Camera Raw | the profile's curve, else ACR3, via RGBTone in ProPhoto | every raw (D21) |
-| Sigmoid | D19's log-logistic curve | — |
+| Sigmoid | D19's log-logistic curve | every raw, for now |
+| Camera Raw | the profile's curve, else ACR3, via RGBTone in ProPhoto | — (D21: the default is open) |
+
+*Amended 2026-10-03:* Camera Raw 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.
 
 A JPEG is still not rendered again (FR-DEV-3j). Film simulation still replaces the view transform
 when a stock is chosen.

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 | Camera Raw tone for raws | **DECIDED 2026-10-03** — the view transform gains a Camera Raw curve (profile's, else ACR3 default, via RGBTone in ProPhoto), the default for every raw; Sigmoid stays a choice |
+| D21 | Camera Raw tone for raws | **PARTLY DECIDED 2026-10-03** — the view transform gains a Camera Raw 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 |
 | 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
@@ -2773,7 +2773,15 @@ 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 — Camera Raw tone for raws · **DECIDED 2026-10-03**
+### D21 — Camera Raw tone for raws · **PARTLY DECIDED 2026-10-03**
+
+*Amended the same day:* the default is **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
+than its Lightroom preview (mean 0.39 against 0.31). The sigmoid stays the default; the curve
+below is a choice; the default is decided by measurement against Lightroom exports of unedited
+photographs (the `lr-fit` work). What follows is the original text.
 
 **The view transform has two curves, and Camera Raw's is the default for every raw.** It is the
 profile's `ProfileToneCurve`, or the ACR3 default curve where the profile has none or there is no