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.