Make the view transform an operation the photographer can set

FR-DEV-3j gives the view transform two controls, contrast and the white
point in stops above middle grey, persisted and held per mask layer like
any other setting. A scene-referred pipeline whose white point cannot be
moved hands the photographer a shoulder they cannot place.

`view_transform` is a hand-written node in `Stage::View`, a new stage
the composer emits last and emits whatever the node's state: a neutral
operation is otherwise left out of the shader, but a photograph with no
view transform is a scan. "Active" keeps meaning "moved from the
defaults", so an untouched photograph writes nothing for it and
`every_node_starts_neutral` still holds. A caller whose chain holds no
view operation gets a default one.

The composer's loop becomes an ordered list of steps — camera nodes, the
matrix, scene nodes, layer-only nodes, the view — so each is emitted in
exactly one place. The base curve could not be a node because it
belonged to the camera; the ops README records why that argument went
with it (D19). The panel shows it in the Light group as "Tone Mapping".

Tests that counted the blocks of a neutral graph now count one, the
view transform, and the two chain-wide tests that load a film expect the
view transform to be absent, since a stock replaces it.

ui/dr-ui/src/labels.rs

@@ -203,12 +203,20 @@ fn catalogued(key: &str) -> Option<&'static str> {
         // it is a tick box, and a checkbox labelled with a verb reads as a
         // button that does something once rather than a state that is on.
         "op.lens_profile" => "Lens Profile",
+        // The view transform (FR-DEV-3j). "Tone Mapping" rather than the
+        // "View Transform" `derive` would give: the id names where it sits in
+        // the pipeline, and the photographer is choosing how the scene's range
+        // is fitted onto the screen.
+        "op.view_transform" => "Tone Mapping",
 
         // Parameters
         // Named for what it does rather than what it is, since a lone
         // parameter is titled by its operation and this one never reaches the
         // panel under its own name — see `rows_filtered`.
         "param.lens_profile.apply" => "Apply",
+        "param.view_transform.contrast" => "Contrast",
+        // In stops above middle grey: where the scene reaches display white.
+        "param.view_transform.white" => "White Point",
         "param.temperature" => "Temperature",
         "param.tint" => "Tint",
         "param.exposure" => "Exposure",