render: table vertical-align and specified column/row sizing

Cells now vertical-align the way tables actually do: middle by default
(the effective UA behavior), with author top/middle/bottom parsed and
applied to whichever shape the cell took: main-axis alignment on the
flex-column stand-in, line alignment on a promoted inline context, or a
pinned-origin offset for the pure-text leaf path where the shaped text
height is known at finalize.

Specified column widths finally apply. <col>/<colgroup> widths (style
or attribute, with span) and colspan-1 cell widths are recorded per
column at build time and pinned inside the column-balancing pass: px
directly, percent against the table's content width, with each column's
content min-content still flooring it. The cell's own width is
neutralized once it has fed the column, so the cell box fills its grid
area instead of shrinking to a fraction of its own already-sized track
(a width:50% cell previously ended up at 25 percent). Unspecified
columns keep content sizing but stretch to fill a definite table width
instead of leaving a dead strip of table background. Row and cell
heights act as row minimums.

Verified against Chromium on a 5-case repro (column from cell width,
row backgrounds, vertical alignment, col element width, percent cell in
a fixed table): all match. Wikipedia and the prose spot set unchanged;
obstacle course 33/33.

crates/obscura-render/src/lib.rs

@@ -320,6 +320,11 @@ pub struct LayoutStyle {
     /// `opacity`, own (non-inherited) value in 0.0-1.0. `None` means the
     /// default of 1.0.
     pub opacity: Option<f32>,
+    /// `vertical-align` for a table cell's content. Cells effectively default
+    /// to `middle` in browsers (the HTML UA sheet sets it on row groups and
+    /// cells inherit it); obscura applies it as main-axis alignment of the
+    /// cell's flex-column stand-in. `None` on non-cell elements.
+    pub vertical_align: Option<VerticalAlign>,
     /// Resolved during the inheritance pass: true when this element should
     /// not be painted at all, either from its own or an inherited
     /// `visibility: hidden`, or because the product of its own and every
@@ -447,6 +452,17 @@ pub enum ObjectFit {
     None,
 }
 
+/// `vertical-align` positions for table-cell content. `baseline` (and the
+/// text-level values like sub/super, which do not apply to cells) map to
+/// `Top` as an approximation: real per-row baseline alignment needs shared
+/// ascent metrics across the row.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum VerticalAlign {
+    Top,
+    Middle,
+    Bottom,
+}
+
 /// `line-height`: `normal` (a font-relative default), a unitless multiple of
 /// font-size, or an absolute pixel length.
 #[derive(Debug, Clone, Copy, PartialEq)]