WIP: noise reduction

Checkpoint committed by the coordinator, not by the authoring agent: the
session hit its API limit mid-task and left this work uncommitted. Committed
so it survives, NOT because it is finished - expect failing tests and
half-applied changes. The agent resumes from here.

core/dr-pipeline/ops/README.md

@@ -211,7 +211,9 @@ 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). `vignetting` is hand-written too but is not in the develop chain — it
+bands), and `noise_reduction` (a kernel, and one that decides how many
+dispatches to emit at each resolution — 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/noise_reduction.yaml

@@ -0,0 +1,30 @@
+# A hand-written node. `rust:` names the type in `crate::ops` that implements
+# `Operation`; its descriptor, its parameters and its passes come from that
+# type rather than from this file.
+#
+# It appears here anyway so that `ops/` lists the whole pipeline in order —
+# including the neighbourhood operations, which run as a group after every
+# point operation but are still ordered among themselves.
+id: noise_reduction
+order: 110
+
+rust: NoiseReduction
+
+why_rust: |
+  A kernel, not four facts. The declarative schema hands a fragment a colour
+  and no coordinate, which is precisely what a denoiser cannot work with — it
+  is defined by what the neighbouring pixels are doing. It is therefore a
+  `DetailStage` (see `../src/detail.rs`), which means deciding at every render
+  how many dispatches to emit, converting a radius stated in *source* pixels
+  into the render pixels this frame is being drawn at, and declaring the halo
+  the tile scheduler needs. None of that is expressible as a uniform
+  expression, and stretching the schema to cover it would produce a worse
+  language than Rust aimed at one caller.
+
+placement: |
+  First among the detail operations, because denoising is a repair and
+  everything else in this stage is an enhancement: sharpening or adding
+  clarity to a noisy frame amplifies the grain along with the detail, and no
+  later pass can separate them again. Its position relative to the point
+  operations is not this number's to decide — the whole detail stage runs
+  after the fused pass, in linear light, before the output transform.