Stop describing a base curve the pipeline no longer has

D19 retired the per-body base curve, moved the matrix ahead of the
edits and the film into the view transform's place, but a dozen doc
comments still listed the curve among what a pixel passes through, or
said the film skipped it. The detail stage's module doc still drew the
matrix after the edits and the last detail pass encoding, which the
view pass took over. The film crate's README gave the base curves as
its reason for being data, and the ops README's list of hand-written
nodes had neither the view transform nor three of the five kernels.

FR-MRG-2 gave the base curve as why the merge cuts below the profile;
the view transform is why now. The decision table still said colour
defaults were a per-body curve, and FR-DEV-3j said only the default
view transform skips a JPEG, where the node skips one whatever its
sliders say. frame-budget.md records the view pass as unmeasured.

core/dr-film/README.md

@@ -1,9 +1,8 @@
 # Film stocks
 
 One file per stock in [`profiles/`](profiles/). Adding a stock is adding a
-file — no code change, no shader, no new operation — for the same reason
-`dr-decode`'s base curves work that way: under the GPLv3 a stock should be
-contributable without a release.
+file — no code change, no shader, no new operation — because under the GPLv3
+a stock should be contributable without a release.
 
 ## What a profile is
 
@@ -51,6 +50,13 @@ matters — see [`src/bake.rs`](src/bake.rs) for the argument:
    log exposure through the negative, the enlarger's exposure is added there,
    and the paper's own curve row and cube take it to linear sRGB.
 
+The stock is the last thing that happens to the picture. It runs in the view
+transform's place (D19): handed linear sRGB, scene-referred, after every other
+adjustment and after sharpening and noise reduction, and handing back the
+rendering the output transform encodes. So every other slider decides the
+exposure the negative receives, and the default tone mapping is not applied
+on top.
+
 Per pixel that is a matrix multiply, a handful of curve taps and one texture
 fetch — two for a print. Splitting 2 from 3, rather than baking one LUT over exposure, is measured
 rather than assumed: the curve carries all the sharp shape and the dye mixing