Add root-level styles to Layout

[nicoburns]

Summary

Adds persistent root-level styles to Layout (linebender#291), so the root style is no longer discarded after building and its font metrics are available even for empty layouts (fixes the invisible-cursor case from linebender#391).

New public type nesting a regular TextStyle:

pub struct RootStyle<'family, 'settings, B: Brush> {
    pub style: TextStyle<'family, 'settings, B>,
    /// Opt-in CSS-like strut: floor every line's metrics by the root font's metrics.
    pub strut: bool,
}
impl From<TextStyle<..>> for RootStyle<..> // strut: false

All three builders now take it (breaking change):

ranged_builder(fcx, text, scale, quantize, root_style: &RootStyle<B>)
style_run_builder(.., root_style: &RootStyle<B>)
tree_builder(fcx, scale, quantize, root_style: &RootStyle<B>) // was &TextStyle

The root style is resolved once, stored on LayoutData, and exposed via:

Layout::root_style() -> &Style<B>
Layout::root_font_size() / root_line_height() -> f32
Layout::root_font_metrics() -> &FontMetrics

Key mechanics:

• FontMetrics::from_font(font, size, synthesis, variations) (new in parley_engine) computes metrics directly via skrifa without shaping; root metrics are resolved with a fontique query using the root style's family/weight/width/style/variations.
• Empty layouts no longer shape a fake " " to get cursor metrics — empty layouts and trailing empty lines (after a final newline) now derive their line-box/content-box metrics from the root font metrics and root line height.
• RangedBuilder::push_default still updates the layout's root style, preserving existing semantics.
• With strut: true, each line's ascent/descent/line-height are floored by the root font's metrics (CSS strut behavior); default remains unchanged.
• Fixed an underflow in analysis when a zero-length style range carried a non-default word_break.

All builder call sites (examples, benches, editor, tests) migrated; new tests cover empty-layout metrics, trailing empty lines, strut flooring, and builder equivalence of the new root fields.

Link to Devin session: https://app.devin.ai/sessions/84c19f0ab14045d6a472af0934ded64c
Requested by: @nicoburns

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