Replace the per-body base curve with a scene-referred view transform

The base curve was a five-point spline on the unit square, flat past its
last point: every value above 1.0 left it as the same number, per
channel. Exposure and highlight recovery put values up there, and the
curve threw them away, then handed the result on as though it were
still scene-linear. The six per-body curves were also, by their own
file's account, hand-tuned shapes rather than measurements, and not
enough is known about where they came from to keep them (D19).

In their place, one view transform for every body (FR-DEV-3j): a
log-logistic sigmoid per channel, with the middle channel put back
between the other two so a hue survives the shoulder. Its two free
constants are solved from two conditions rather than set: scene grey
0.13, where the retired default curve put it, lands on display 0.18,
and the scene white four stops above grey lands on 1.0. So a highlight
a stop past sensor saturation still rolls into white, and the midtones
stay within 0.26 EV of the retired default between scene 0.03 and 1.0.
`dr_pipeline::view` holds the CPU reference and the WGSL, and the tests
there are FR-DEV-3j's acceptance criteria.

It is still fixed and still in the fused pass's tail, so a detail stage
still sees rendered values; the next commits make it an operation and
move it after the detail stage. It is skipped for a JPEG, as the base
curve was, and absent from the camera-space tap.

The base curve's database, its lookup and its twelve uniform slots go.
`RawImage` and `DemosaicedImage` lose the field, and the GPU test that
proved a curve reached the shader is replaced by one that renders the
view transform against the CPU reference and shows two highlights above
1.0 still render apart. The JPEG-and-sensor test now asserts the two
differ by exactly the view transform, where before an identity fixture
curve had made them match.

docs/dev/architecture.md

@@ -452,7 +452,7 @@ exactly the stall §6.1 exists to prevent.
 **Two reductions, not one.** The display histogram (FR-DSP-7) counts the frame the output transform
 produced: its axis is the output code value, and a clipped bin means a highlight that is gone as the
 image currently stands. The raw histogram for culling (FR-CULL-3) counts the **demosaiced
-scene-linear texture** — before white balance, the camera matrix, the base curve and the tone chain
+scene-linear texture** — before white balance, the camera matrix, the tone chain and the view transform
 — on an axis of stops below sensor saturation, which is how it reports headroom the embedded JPEG's
 histogram cannot. A culling decision needs the second, an export decision needs the first, and
 neither answers for the other. Both are drawn by the same panel and chosen between.