Sync with integration

core/dr-pipeline/ops/README.md

@@ -209,11 +209,12 @@ They still belong in this directory, because the pipeline's **order** is the
 one thing a reader comes here to learn, and an order written half in YAML and
 half in Rust would be worse than either alone.
 
-Currently hand-written: `tone_curve` (a curve widget over five interpolated
-points), `colour_mixer` (thirty-six faceted parameters from twelve computed hue
-bands), `capture_sharpen` (a separable convolution, which is the other reason a
-node is Rust — see the next section). `vignetting` is hand-written too but is
-not in the develop chain — it
+Currently hand-written: `tone_curve` (one widget over four curves of five
+interpolated points — master, red, green, blue — each reaching the shader only
+when it has been moved), `colour_mixer` (thirty-six faceted parameters from
+twelve computed hue bands), `capture_sharpen` (a separable convolution, which
+is the other reason a node is Rust — see the next section). `vignetting` is
+hand-written too but is not in the develop chain — it
 carries lens-profile coefficients that are not parameters. `distortion` and
 `aberration` are `Warp`s rather than operations: they rewrite coordinates
 before sampling rather than transforming a colour after it.

core/dr-pipeline/ops/tone_curve.yaml

@@ -16,13 +16,26 @@ attributes: [tone, colour]
 rust: ToneCurve
 
 why_rust: |
-  Five control points presented as one curve widget, with an interpolator and
-  a monotonicity guarantee behind it. Its neutral is a *relationship* between
-  parameters rather than a set of values — the identity diagonal — which is
-  not something the declarative `active:` rule can express, and its
-  `presentation()` spans parameters rather than describing one.
+  Four curves — master, red, green, blue — of five control points each,
+  presented as one widget, with an interpolator and a monotonicity guarantee
+  behind them. Its neutral is a *relationship* between parameters rather than
+  a set of values — the identity diagonal, on every channel — which is not
+  something the declarative `active:` rule can express; its `presentation()`
+  spans forty parameters rather than describing one; and its fragment is
+  *assembled* rather than written, because each curve reaches the shader only
+  when it has been moved off the diagonal. A declared node's `wgsl:` is one
+  fixed block of text, which is the right shape for nearly everything here and
+  the wrong one for a node whose cost has to follow what the photographer
+  actually touched.
 
 placement: |
   After the fixed-weight region controls, so the curve is the final word on
   tone: a photographer reaches for it to fix what those controls could not
   place exactly.
+
+  Within the node, the master curve runs before the per-channel ones. Both
+  orders are visibly different images and the reasons for this one are written
+  out in `src/ops/curve.rs`: the master is tonal and hue-preserving, the
+  per-channel curves are the chromatic grade over the tones it produced, and a
+  point placed on a channel curve should act on the tone the photographer can
+  see rather than on the one the master is about to move.