`Cap::Butt` is rendered wrong

Nobody has claimed this yet.

```rust
let stroke = Stroke {
width: 50.0,
join: vello::kurbo::Join::Miter,
miter_limit: 10.,
start_cap: vello::kurbo::Cap::Butt,
end_cap: vello::kurbo::Cap::Butt,
..Default::default()
};
let mut path = BezPath::new();
path.move_to(Point::new(110., 50.));
path.line_to(Point::new(110., 60.));
path.line_to(Point::new(100., 60.));
scene.stroke(&stroke, Affine::IDENTITY, Color::WHITE, None, &path);
```
makes this:


but html 2d canvas:


using this code

```html

const canvas = document.getElementById('canvas');
const ctx = canvas.getContext('2d');
ctx.fillStyle = '#000';
ctx.fillRect(0, 0, 500, 300);

ctx.lineWidth = 50;
//ctx.lineJoin = 'bevel';
//ctx.lineCap = "round";

ctx.strokeStyle = '#fff';
ctx.beginPath();
ctx.moveTo(110, 50);
ctx.lineTo(110, 60);
ctx.lineTo(100, 60);
ctx.stroke();

```

Start by reproducing the issue with the Rust Stroke and BezPath snippet, then compare its Cap::Butt output with the HTML canvas example. Trace the scene.stroke path and stroke-cap handling to find where the rendered geometry diverges. Done means the same path renders with butt caps matching the canvas result.

Written by the indexing model from the issue text.