The P1 gate is a pixel comparison, so it needs to be a command rather than a
paragraph. `tools/verify-screen` renders each screen in the manifest both ways
and reports the largest per-channel difference anywhere in the frame.
Both renderers are held to the same inputs on purpose:
* the reference CLI is the one `build-reference-cli` builds from the revision
the exporter is PINNED to -- not `/reborn/target/`, which is a live mount of
the other agent's checkout and moves mid-iteration. A pixel disagreement
against a moving decoder has a free variable in it and proves nothing.
* `--black`, because the screen carries its own background and the CLI's default
dim slate stands in for a 3D scene that is not on this screen.
* `--primitives --animated`, because the port draws every element at rest and
those two flags are what make the CLI draw the same set. NOT `--focus`:
nothing is focused at rest.
The threshold is 3/255 -- what integer-truncating compositing in the CLI and
float rounding on a GPU differ by. Above that is a placement, order or colour
disagreement that needs a reason, and the script says DIFFERS rather than
pretending a wider tolerance is a result.
