Screenshot tests

What this backend can prove about pixels, on a laptop, with no panel.

../README.md is the front page.

[dev-dependencies]
xpui-screenshot = { git = "https://github.com/XPUI-Framework/xpui-backends", branch = "main", features = ["golden"] }

A separate crate, because it is std, it writes files, and this one builds for bare metal. Framebuffer is a plain 1-bit DrawTarget with no window and no hardware, so a screen can be rendered and asserted on in an ordinary cargo test:

backend.with_display(|frame| {
    assert_screenshot("my_screen", frame);      // against a committed PNG
    assert!(frame.ink_in(0, 0, 480, 56) > 0);   // and what a picture cannot say
});

assert_screenshot compares the whole panel against tests/screenshots/my_screen.png in the crate being tested, pixel for pixel. The first run writes the golden and fails, so that nobody commits a picture they never looked at; UPDATE_SNAPSHOTS=1 rewrites it afterwards. A mismatch prints an ASCII view marking every block that changed and writes target/diff/my_screen.png — expected, actual and the differences, side by side.

That is how this crate's own tests work — see tests/screenshots.rs.

check_screenshot is the same comparison handing back its report as Result<(), String> rather than panicking with it, for a test that captures many frames and wants to name every one that moved rather than stopping at the first. A broken harness — a golden that will not decode, a directory that will not take a file — still panics through either of them. examples/gallery renders nine screens on seven boards that way, so one token moved by one pixel names every board it reached rather than the first.

write_bmp and thumbnail are still there, and are for looking at a frame rather than asserting on one. Nothing compares them.