Inconsistent baseline difference line-to-line for some fonts/sizes

parley = { version = "0.8.0" }

When experimenting with B612 Mono in a text box UI widget I'm building, I noticed the text wobbling up and down during line scrolling. It looks like the change in baseline from line to line is not always consistent for this font, though I'm not sure what it is about this font that makes it that way.

Repro:

use parley::{FontContext, FontFamily, LayoutContext, StyleProperty};

const TEXT: &str = "|\n|\n|\n|\n|\n|\n|\n|\n|\n|\n|\n|\n|\n|\n|\n|\n|\n";

pub fn repro() {
    let mut font = FontContext::new();
    font.collection.load_system_fonts();
    font.collection.register_fonts(
        include_bytes!("path/to/B612Mono-Bold.ttf").to_vec().into(),
        None,
    );

    print_baselines(&mut font, "Segoe UI", TEXT);
    print_baselines(&mut font, "B612 Mono", TEXT);
}

fn print_baselines(font: &mut FontContext, family: &'static str, text: &'static str) {
    let mut layout = LayoutContext::<()>::new();

    let mut builder = layout.ranged_builder(font, text, 1.0, false);

    builder.push_default(StyleProperty::FontSize(14.0));
    builder.push_default(StyleProperty::FontFamily(FontFamily::Source(family.into())));

    let mut layout = builder.build(text);

    layout.break_all_lines(None);

    let mut last = 0.0;
    let baselines = layout
        .lines()
        .map(|l| {
            let baseline = l.metrics().baseline;
            let delta = baseline - last;
            last = baseline;
            (baseline, delta)
        })
        .collect::<Vec<_>>();

    println!(
        "{family}: {}",
        baselines.iter().map(|(b, d)| format!("({b}, +{d})")).collect::<Vec<_>>().join(", ")
    );
}

Output:

Segoe UI: (15.107422, +15.107422), (33.728516, +18.621094), (52.34961, +18.621094), (70.9707, +18.621094), (89.5918, +18.621094), (108.21289, +18.621094), (126.833984, +18.621094), (145.45508, +18.621094), (164.07617, +18.621094), (182.69727, +18.621094), (201.31836, +18.621094), (219.93945, +18.621094), (238.56055, +18.621094), (257.18164, +18.621094), (275.80273, +18.621094), (294.42383, +18.621094), (313.04492, +18.621094), (331.66602, +18.621094)
B612 Mono: (13.51, +13.51), (30.52, +17.01), (47.53, +17.009998), (64.54, +17.010002), (81.55, +17.010002), (98.560005, +17.010002), (115.57, +17.009995), (132.58, +17.010002), (149.59, +17.009995), (166.59999, +17.009995), (183.61, +17.01001), (200.62, +17.009995), (217.62999, +17.009995), (234.64, +17.01001), (251.65, +17.009995), (268.66, +17.01001), (285.67, +17.01001), (302.68002, +17.01001)

Notice the 17.01, 17.009998, 17.010002 etc. delta values for B612 Mono compared to the very consistent 18.621094 value for Segoe UI. This is causing some jittery pixel offsets when I go to quantize in some situations. I'm not sure if this is a consequence of #273 or a separate issue.

Start with the provided Rust reproduction, focusing on LayoutContext::ranged_builder, layout.lines(), and each line's metrics().baseline. Trace how baseline values are calculated for B612 Mono versus Segoe UI and capture the case in a regression test; done means repeated line deltas no longer produce the reported inconsistent offsets.