Tables: Implement per-cell alignment and style specifiers

[nlopes]

Problem

The full AsciiDoc cell specifier syntax supports per-cell alignment and style overrides, not just spans.

AsciiDoc syntax

|===
| Normal | ^| Centered | >| Right-aligned
| Default | .^| Vertically centered | e| Emphasis style
| 2+^.>m| Colspan 2, center, bottom, monospace
|===

Cell specifier format: [colspan][.rowspan]+[halign][.valign][style]|

Where:

• halign: < (left), ^ (center), > (right)
• valign: .< (top), .^ (middle), .> (bottom)
• style: a (asciidoc), d (default), e (emphasis), h (header), l (literal), m (monospace), s (strong)

Current behavior

Only span specifiers (2+, .3+, 2.3+) are parsed. Alignment and style modifiers are treated as cell content.

Implementation requirements

1. Parser changes (acdc-parser/src/blocks/table.rs)

Extend CellSpecifier to include:

pub struct CellSpecifier {
    pub colspan: usize,
    pub rowspan: usize,
    pub halign: Option<HorizontalAlignment>,  // NEW
    pub valign: Option<VerticalAlignment>,    // NEW
    pub style: Option<ColumnStyle>,           // NEW
}

2. AST changes (acdc-parser/src/model/tables.rs)

Add optional per-cell formatting to TableColumn:

pub struct TableColumn {
    pub content: Vec<Block>,
    pub colspan: usize,
    pub rowspan: usize,
    pub halign: Option<HorizontalAlignment>,  // NEW: overrides column default
    pub valign: Option<VerticalAlignment>,    // NEW: overrides column default
    pub style: Option<ColumnStyle>,           // NEW: overrides column default
}

3. HTML renderer (converters/html/src/table.rs)

Use per-cell alignment/style if specified, otherwise fall back to column defaults.

Priority

Low - column-level alignment works for most use cases. Per-cell overrides are less common.

References

• https://docs.asciidoctor.org/asciidoc/latest/tables/format-cell-content/
• https://docs.asciidoctor.org/asciidoc/latest/tables/align-by-cell/

Related

• Follow-up from Tables: Implement cell spanning (colspan/rowspan) #276