Develop the film last, in the view transform's place

A film stock ran at order 25, after white balance and exposure, and
everything after it — contrast, the curves, the colour mixer, the
grading, every mask layer's tone — acted on the film's display-referred
output, as though the frame had been scanned and then worked on. That
was a display-referred rendering in the middle of the chain, which D19
removes (FR-DEV-3f, FR-DEV-3j).

`film_sim` is now in `Stage::View` beside `view_transform`. While a
stock is loaded the composer emits it in the view transform's place and
not the sigmoid; otherwise the sigmoid. So every edit is a decision
about the exposure the negative receives, and the film is the last
thing that happens to the picture — after the detail stage too, in the
view pass, which already binds the film's tables, the mask array and
the grain's source position. Its per-layer settings blend there as they
did in the fused pass.

`op_renders` goes: a rendering is chosen, not suppressed, and the only
render with no view transform is the camera-space tap. The YAML order
moves to 190 so the panel reads in pipeline order; the stage, not the
number, is what places it.

Existing edits that combine a stock with tone or colour operations now
render differently: those operations used to act on the print, and now
act on the scene.

core/dr-pipeline/ops/README.md

@@ -240,17 +240,16 @@ coefficients that are not parameters.
 `Operation::renders`, and it is worth knowing why before writing a second one.
 
 Every other node *adjusts* a picture. That one *makes* it: a film stock's
-characteristic curve does the camera profile's base curve's job, from
-measurements rather than from a curve somebody drew. Running both renders the
-scene twice — the camera's rendering, and then a film's rendering of *that* —
-which looks like neither and reads as a colour-management bug with no
-colour-management bug to find.
+characteristic curve does the view transform's job, from measurements rather
+than from a curve somebody chose. Running both renders the scene twice — the
+default rendering, and then a film's rendering of *that* — which looks like
+neither and reads as a colour-management bug with no colour-management bug to
+find.
 
-So a node declaring `renders` hands back display-referred linear sRGB, and in
-exchange the composer does not emit the base curve. It is handed working-space
-colour like every other node: the conversion out of camera space is no longer
-the rendering's to take over, because since D19 it runs before every node but
-white balance (see "Stages" below). `distortion` and
+So `film_sim` is in `Stage::View` beside `view_transform`, and while a stock
+is loaded the composer emits it in the view transform's place, last, after the
+detail stage, and not the sigmoid (D19). It is handed working-space colour and
+hands back display-referred linear sRGB for the output transform. `distortion` and
 `aberration` are `Warp`s rather than operations: they rewrite coordinates
 before sampling rather than transforming a colour after it.