Keep trailing spaces on their line; expose Style::parent and Line::style_baseline

[nicoburns]

LLM Contributions: Devin (Cognition) authored the code and this description under human direction.

Replaces #20. Stacked on #18 → #19, which are now themselves based on linebender#762 (1218f0d), so this PR contains only the fixes below.

Trailing spaces (line_break.rs)

linebender#762 decides which trailing whitespace hangs (in finish_line), but the breaker still unconditionally start_new_lines after appending an overflowing hanging space. Consequences: "AAA " at the end of the layout produces a second, empty line (layout.len() == 2, still reproducible on linebender#762 and upstream main), and "XXXX XX" under pre-wrap breaks after the first hanging space instead of hanging both.

Now, after appending an overflowing hanging space, the breaker continues while the next character in the layout is another hangable space (U+0020 or tab — matching linebender#762's is_space set), or when there is nothing after it; otherwise it starts the new line as before. The lookahead uses the global character list so it also works when the spaces are split across runs (a style change inside the trailing spaces), and an inline box following the space still breaks. NBSP is not chained (it doesn't hang under linebender#762's rules; the nbsp snapshot is updated for that).

Regression test: trailing_whitespace_does_not_add_line (single space, multiple spaces, NBSP, hard wrap, soft wrap, and two cases with a font-size change on the middle of three trailing spaces at a width where the first space already overflows).

Public Style::parent() and Line::style_baseline()

Renderers need to draw text decorations at the baseline of the decorating inline box, not at the (possibly shifted) baseline of each glyph run. Line::style_baseline(style_index) was pub(crate); it is now pub, plus a Style::parent() accessor so the renderer can walk from a run's style to the style that owns the decoration. Test: vertical_align_style_baseline.

Blitz WPT (8 dirs, 4188 run) with this pin: 2533 pass vs 2523 for #20 — the +10 is linebender#762's own gains (justification / trailing-space tests) minus the two line-break-anywhere-overrides-uax-behavior-004/006 accidental passes linebender#762's description already notes.

Changelog

Added

• Style::parent() and Line::style_baseline() expose the inline style tree and per-style baselines for decoration painting.

Fixed

• Trailing spaces no longer create an empty final line, and consecutive overflowing spaces hang on the same line.

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/da7669e341814cc7976e4ebf32b7eacb
Open in Devin Desktop: https://dioxus.staging.devinenterprise.com/desktop/session/da7669e341814cc7976e4ebf32b7eacb?variant=devin-insiders
Requested by: @nicoburns

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