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.

core/dr-pipeline/ops/view_transform.yaml

@@ -0,0 +1,18 @@
+id: view_transform
+order: 200
+# A `rust:` node publishes its own descriptor; its attributes are on the type
+# in `../src/ops/view_transform.rs`.
+rust: ViewTransform
+
+why_rust: |
+  It is composed at its defaults — a photograph with no view transform is a
+  scan, not a picture — which is `Stage::View`, and a declaration has no way to
+  say it. Its three uniforms are also the solution of two equations rather than
+  expressions over its parameters (`dr_pipeline::view::Sigmoid::new`).
+
+placement: |
+  Last, after every scene operation and, when there is one, after the detail
+  stage (D19, FR-DEV-3j). It is the one stage allowed to map scene-linear colour
+  to a display range, so anything after it would be working on a rendering.
+  The order here only places it in the panel; the composer puts every
+  `Stage::View` node at the end whatever its number says.