Implement CSS vertical-align for inline formatting contexts

[nicoburns]

LLM Contributions: Written with Devin (Cognition AI); design and review by the author.

Implements the CSS 2.2 §10.8 inline alignment model. Commit-by-commit:

1. Style tree parents — TreeStyleBuilder now materialises ancestor styles (even ones with no direct text) into the style table, parent-first, and each Style carries parent: u16. Root style is index 0 and is the strut. RangedBuilder styles all parent 0.
2. VerticalAlign — new StyleProperty::VerticalAlign(VerticalAlign) and InlineBox::vertical_align. Following css-inline-3, VerticalAlign { alignment: AlignmentBaseline, shift: BaselineShift } is a compound of the two longhands (AlignmentBaseline::{Baseline, TextTop, TextBottom, Middle}, BaselineShift::{Length(f32), Sub, Super, Top, Bottom}), so vertical-align: text-top 2px is representable and callers with the longhands (Blitz/Stylo) pass them through losslessly. The CSS 2 keywords are associated constants (VerticalAlign::SUPER, ::TEXT_TOP, …) plus VerticalAlign::length(px). Percentages must be resolved by the caller.
3. StyleMetrics pass (layout/style_metrics.rs) — once per layout, per style: primary-font ascent/descent/x-height (FontMetrics::from_variations), line-height as half-leading → over/under, then baseline_offset relative to the parent = alignment offset (middle uses the parent x-height and the child's expanded box; text-top/bottom the parent content box) + shift (sub/super use WebKit's fontSize/5, fontSize/3), and aligned_subtree (nearest top/bottom ancestor, else 0).
4. Line breaking — LineBoxMetrics becomes SmallVec<SubtreeExtents> + a per-line "contributed styles" set. Each line is seeded with the strut; each run/inline box adds its own style box at its baseline offset plus any not-yet-contributed ancestors. top/bottom subtrees are sized separately and, in finish_line, grow the line below/above when taller than the root subtree; their offsets are stored on LineData::aligned_subtree_offsets. Lines containing only empty inline boxes stay zero-height.
5. Output — GlyphRun::baseline and PositionedInlineBox::y use the style's shifted baseline via Line::style_baseline / Line::inline_box_top. LineMetrics::baseline stays the root baseline; cursor/selection use whole-line block extents and are unaffected.

Behaviour changes for existing users: lines containing only inline boxes (or none of the text's styles) now get strut height, and a span's inline box uses the primary-font metrics of that style (six snapshots updated). New styles/vertical_align snapshot tests cover every keyword, nested super, ancestor-only spans, inline boxes and tall top/bottom subtrees; unit tests cover the metrics pass including composition of alignment + shift.

Blitz follow-up: DioxusLabs/blitz#832.

Changelog

Added

• StyleProperty::VerticalAlign / VerticalAlign { alignment: AlignmentBaseline, shift: BaselineShift } and InlineBox::vertical_align, implementing CSS vertical-align (parent-relative and line-relative values).

Changed

• Breaking change: InlineBox has a new vertical_align field.
• Line boxes now follow the CSS 2.2 inline-box model more closely: every line includes the root style's strut, and nested tree-builder spans contribute their own inline boxes even when they have no direct text.

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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