Develop longer, from the measurements rather than from a contrast slider

Pushing was not a thing to simulate. It was measured data being thrown
away: Double-X and 2302 each ship five characteristic curves, one per
development time, and this shipped the 6.5-minute column and discarded
four. All five now ship and interpolate.

The axis is real. Double-X runs 4 to 12 minutes, and across it the average
gradient goes 0.472 to 1.034 while Dmax goes 1.19 to 2.56.

The control is in stops, because that is what a photographer means, and one
stop is a factor of about 1.41 in time. That mapping is checked rather than
assumed: against Double-X's own axis it lands within 2% of the 9-minute
column for +1, and near 12 minutes for +2, which are the times the datasheet
gives for exactly that. There is a test.

**Pushing must not recover shadow detail, and this does not.** Across the
whole measured range the speed point moves about a third of a stop while the
gradient doubles; three stops under mid-grey, density goes from 0.008 to
0.035, which is still nothing. Developing longer multiplies what was already
recorded and cannot record what never hit the film. A push built as added
exposure or global contrast brightens those shadows instead and looks
convincing until someone who shoots film sees it, so that property has a
test of its own.

Interpolated in *log* time, because development is multiplicative: 4 to 5
minutes is the same amount of push as 9 to 12, and interpolating linearly
would bunch the control at one end. Clamped at both ends, because past the
published range there is no data and extrapolating a contrast curve invents
an emulsion nobody tested. A stock measured at one process ignores the
control entirely rather than inventing a curve for it -- Portra 800's pushes
are separate *measured* profiles, which is the honest way to offer those.

Costs nothing per pixel and changes no shader. The curves are a per-stock
table, so the interpolation happens on the CPU at bake time, where choosing a
stock and moving its sliders already rebakes. The Vulkan shader is untouched.

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

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

@@ -36,6 +36,7 @@ use crate::operation::{Operation, Uniform};
 pub const ID: OpId = OpId("film_sim");
 pub const EXPOSURE: ParamId = ParamId("exposure");
 pub const PRINT_EXPOSURE: ParamId = ParamId("print_exposure");
+pub const PUSH: ParamId = ParamId("push");
 
 /// How many samples a characteristic curve carries.
 ///
@@ -64,6 +65,11 @@ static DESCRIPTOR: OpDescriptor = OpDescriptor {
     params: &[
         ParamDescriptor::stops("exposure", "param.film_sim.exposure", -3.0, 3.0),
         ParamDescriptor::stops("print_exposure", "param.film_sim.print_exposure", -3.0, 3.0),
+        // TRACES: FR-DEV-3f
+        // Development, in stops of push. Bounded by what the manufacturers
+        // actually published: Double-X's measured axis spans about -1 to +2,
+        // and beyond a range like that a curve would have to be invented.
+        ParamDescriptor::stops("push", "param.film_sim.push", -1.0, 3.0),
     ],
 };
 
@@ -122,6 +128,7 @@ impl FilmTables {
 pub struct FilmSim {
     exposure: f32,
     print_exposure: f32,
+    push: f32,
     tables: Option<FilmTables>,
 }
 
@@ -165,6 +172,7 @@ impl Operation for FilmSim {
         match id {
             EXPOSURE => self.exposure = value,
             PRINT_EXPOSURE => self.print_exposure = value,
+            PUSH => self.push = value,
             _ => log::warn!("film_sim: unknown parameter {id}"),
         }
     }
@@ -173,6 +181,7 @@ impl Operation for FilmSim {
         match id {
             EXPOSURE => self.exposure,
             PRINT_EXPOSURE => self.print_exposure,
+            PUSH => self.push,
             _ => 0.0,
         }
     }