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/src/ops/film_sim.rs

@@ -1,22 +1,25 @@
 //! TRACES: FR-DEV-3f
 //! Film simulation — the stock renders the picture.
 //!
-//! # Why this one replaces the base curve
+//! # Why this one is the view transform
 //!
-//! [`crate::ops`]' other nodes adjust a picture. This one *makes* it. The base
-//! curve exists because sensor data is scene-referred and nothing anybody looks
-//! at is (FR-DEV-3e); a film stock's characteristic curve does the same job,
-//! from measurements, with a toe and a shoulder that were coated onto acetate
-//! rather than drawn. Running both renders the image twice — the camera's
-//! JPEG-ish rendering, and then a film's rendering of that — which is not what
+//! [`crate::ops`]' other nodes adjust a picture. This one *makes* it. The view
+//! transform exists because sensor data is scene-referred and nothing anybody
+//! looks at is (FR-DEV-3j); a film stock's characteristic curve does the same
+//! job, from measurements, with a toe and a shoulder that were coated onto
+//! acetate rather than drawn. Running both renders the image twice — the
+//! default rendering, and then a film's rendering of that — which is not what
 //! either is for and looks like neither.
 //!
-//! So this node declares [`Operation::renders`], and the composer answers by
-//! emitting neither the base curve nor the camera matrix. Both jobs move here:
-//! the fragment takes camera RGB, converts it to linear sRGB itself with the
-//! matrix already in the uniform block, and returns linear sRGB. That is a
-//! contract worth stating plainly, because a node that got half of it wrong
-//! would produce a picture that renders perfectly and is wrong everywhere.
+//! So this node is in [`Stage::View`] and declares [`Operation::renders`]: when
+//! a stock is loaded the composer puts it at the end of the chain in place of
+//! the default sigmoid (D19). It is handed working-space colour — linear sRGB
+//! primaries, scene-referred, after every other operation and after the detail
+//! stage — and returns display-referred linear sRGB for the output transform.
+//! Before D19 it ran at order 25, after exposure and before everything else,
+//! and the operations below it acted on its output. They now act on the scene
+//! it is shown: an edit is a decision about the exposure the negative
+//! receives, and the film is the last thing that happens to the picture.
 //!
 //! # Why the tables are not parameters
 //!
@@ -34,7 +37,7 @@
 use std::sync::{Arc, LazyLock};
 
 use crate::descriptor::{Attribute, LocalizedKey, OpDescriptor, OpId, ParamDescriptor, ParamId};
-use crate::operation::{Operation, Uniform};
+use crate::operation::{Operation, Stage, Uniform};
 
 pub const ID: OpId = OpId("film_sim");
 pub const EXPOSURE: ParamId = ParamId("exposure");
@@ -304,11 +307,17 @@ impl Operation for FilmSim {
         self.tables.is_some()
     }
 
-    /// This node renders; the camera's own rendering must not also run.
+    /// This node renders; the default view transform must not also run.
     fn renders(&self) -> bool {
         true
     }
 
+    /// TRACES: FR-DEV-3f | FR-DEV-3j
+    /// The view transform's place, at the end of the chain (D19).
+    fn stage(&self) -> Stage {
+        Stage::View
+    }
+
     fn set_film_tables(&mut self, tables: Option<&FilmTables>) {
         self.set_tables(tables.cloned());
     }