Render a film stock on the GPU, and let it take over the rendering

The stock model landed in dr-film with no way to see it. This is the
pipeline node, the two texture bindings it reads, and the end-to-end test
that proves the shader agrees with the model.

The design point is that a film simulation is not an adjustment. Every
other node changes a picture; this one makes it. A stock's characteristic
curve does the camera profile's base curve's job -- from measurements
rather than from a curve somebody drew -- so running both renders the
scene twice: the camera's rendering, and then a film's rendering of that.
It looks like neither, and it reads as a colour-management bug with no
colour-management bug to find.

So `Operation::renders` is new. A node declaring it takes camera RGB and
hands back linear sRGB, and the composer emits neither the base curve nor
the conversion out of camera space. Both halves move together, and the
composer keeps them as one string precisely so that getting half of it
right is impossible.

The tables are not parameters, for the reason vignetting's coefficients
are not: they are measurements. dr-pipeline declares the layout as a plain
struct and keeps its no-dependency property; the two crates share no types
on purpose. `EditGraph::set_film_tables` offers them to every node rather
than to the one that wants them, because knowing which concrete type is
which is what the graph is organised not to know.

Bindings 4 and 5 follow the masks precedent: declared unconditionally so
one bind group layout serves every generated shader, bound to 1x1
placeholders when no stock is loaded. Both are interpolated by hand with
textureLoad -- this pipeline binds no sampler, and adding one for two
lookups would cost a binding in every shader. Uploads are keyed on content
so an unchanged stock does not push half a megabyte across the bus per
frame.

The end-to-end test earned its place immediately: it found the density
lookup being filled z-fastest while a 3D texture upload wants x-fastest,
so the red and blue axes were transposed. Green matched exactly, which is
what that bug looks like -- a plausible photograph of the wrong colour,
and one that every unit test on either side of the seam passes. dr-film
now pins the layout in a test that needs no device, and states it where
the field is declared.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/requirements.md

@@ -326,6 +326,29 @@ own JPEG. ΔE2000 validation against ColorChecker references applies once DCP su
 simulation ecosystem at near-zero implementation cost, plus reading the in-RAF film simulation tag
 to auto-apply a matching render for Fujifilm files.
 
+**Spectral film simulation, in addition rather than instead** (`dr-film`). Where a stock's
+measurements exist, simulate the physics instead of replaying a grade: spectral sensitivity exposes
+three emulsion layers, characteristic curves develop them to densities, dye densities absorb, and a
+paper profile prints the negative with the enlarger's filtration solved rather than dialled. A
+scanned negative is therefore orange and inverted, because that is what a negative is.
+
+Two things this buys that a LUT cannot. The parameters stay **physical** — opening up a stop moves
+the picture along the film's own curve, shoulder and all, rather than scaling a number baked at one
+exposure. And the **data cost inverts**: a stock is ~17 kB of published measurements where one
+HaldCLUT is ~800 kB of one person's grade.
+
+A film simulation is a *rendering*, not an adjustment, so it replaces the camera profile's base
+curve and the conversion out of camera space (`Operation::renders`) — applying both would render
+the scene twice.
+
+*Acceptance:* a neutral scene printed through a colour negative's own paper renders neutral to
+within 0.06 in linear sRGB; the baked lookup's interpolation error stays under one 8-bit code
+value; and the shader agrees with the CPU model, which agrees in turn with an independent
+reference implementation.
+
+**Open:** how the chosen stock persists. Sidecar parameters are `f32` and the stock list is
+data-driven, so neither an index nor a name fits the existing shape.
+
 **FR-DEV-3g — AI denoise.** Learned denoising operating in the raw domain, ideally jointly with
 demosaic.