Connect the lens vignetting correction to the pipeline

`ops/vignetting.rs` has carried a complete descriptor, polynomial, helper
and test suite without an entry in `ops/`, so it was never in `chain()`.
It reached no photograph and no panel, and `Attribute::Optics` was an empty
category in consequence — filtered out of the tab strip for having no rows,
by a chain that had never been given its only member.

Declaring it needs the one thing the operation was written against and
which did not exist. `wgsl_body` reads `radius`, and the module claimed
"the composer publishes `radius` in the shader prologue for exactly this
reason". It did not. `sample_source` now does, in both sampling branches,
beside the `source_px` it already published for the same class of caller.

It is corner-normalised there, which is the part that is easy to leave out.
`p` spans ±0.5·aspect, so its length at the corner is 0.5·length(aspect) —
about 0.901 on a 3:2 frame, not 1. Lensfun's polynomials are fitted against
a corner radius of 1, so passing `length(p)` straight in evaluates every one
of them short of where it was measured, by a factor that changes with the
aspect ratio. It would have read as a correction that is simply too weak,
which is indistinguishable from a bad profile. Both `lens.rs` and
`framing.rs` asserted the normalisation `p` does not have; corrected.

`order: 5` puts the correction ahead of the tonal stages, and the ordering
is load-bearing rather than tidy. Recovering a corner means dividing by an
attenuation below one — about two stops for a fast prime wide open — so run
after the highlights have been rolled off and clipped, the lift has nowhere
to go and the corners posterise instead of brightening.

`layer_chain` now drops `Optics` as well as the neighbourhood operations.
A local vignetting slider would have worked, which is what makes it worth
excluding: `radius` measures from the centre of the whole photograph and a
mask cannot move the optical axis, so it would lay a frame-centred radial
ramp across the picture and multiply it by the mask. The existing exclusion
covers operations that move and do nothing; this one covers an operation
that moves and does something its name does not promise. The rule both
share is now written down.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

core/dr-pipeline/src/ops/mod.rs

@@ -54,6 +54,14 @@
 //! than `Operation`, because they rewrite *coordinates* before the source is
 //! sampled rather than transforming a colour after it. They are not part of
 //! the develop chain and do not appear in `ops/`.
+//!
+//! [`vignetting`] is the exception, and the reason the split is drawn at
+//! coordinates rather than at "lens correction": it applies a gain to the
+//! pixel already fetched, so it is an ordinary node in `ops/` like any other.
+//! What it needs that a colour fragment is not otherwise given is the pixel's
+//! distance from the optical axis, which the sampler publishes as `radius`
+//! — corner-normalised there, because the profile coefficients are fitted
+//! against a corner radius of 1 and the prologue's `p` is not.
 
 // Hand-written nodes. Each is listed in `ops/` with `rust:`, which is what
 // places it in the chain; these are the implementations that entry points at.