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-film/src/bake.rs

@@ -59,6 +59,13 @@ pub struct Recipe<'a> {
     pub exposure_ev: f32,
     /// Enlarger exposure, in stops. Ignored without a `print`.
     pub print_exposure_ev: f32,
+    /// TRACES: FR-DEV-3f
+    /// Development, in stops of push. Positive develops longer.
+    ///
+    /// Ignored by a stock measured at one process, of which there are many —
+    /// see [`crate::profile::Profile::curves_at_push`], which returns the one
+    /// measured curve rather than inventing a pushed one.
+    pub push_stops: f32,
 }
 
 impl<'a> Recipe<'a> {
@@ -70,6 +77,7 @@ impl<'a> Recipe<'a> {
             print,
             exposure_ev: 0.0,
             print_exposure_ev: 0.0,
+            push_stops: 0.0,
         }
     }
 }
@@ -260,8 +268,17 @@ pub fn bake(recipe: &Recipe) -> Baked {
         }
     }
 
-    let curves = film.density_curves.clone();
-    let density_max = film.max_density().max(1e-3);
+    // TRACES: FR-DEV-3f
+    // Developed to the requested push before anything else reads the curves:
+    // the density ceiling, the print balance and the grain all depend on how
+    // far this film was taken, and a push that only reached one of them would
+    // be a contrast change wearing a push's name.
+    let curves = film.curves_at_push(recipe.push_stops);
+    let density_max = curves
+        .iter()
+        .flat_map(|row| row.iter())
+        .fold(0.0f32, |a, &b| a.max(b))
+        .max(1e-3);
 
     let viewing = match recipe.print {
         Some(paper) => Viewing::new(&paper.viewing_illuminant),