Tables: Implement cell spanning (colspan/rowspan)

[nlopes]

Problem

Table cell spanning syntax is not implemented. AsciiDoc supports colspan and rowspan via special syntax in table cells.

AsciiDoc syntax

[cols="3*"]
|===
| Cell 1 | Cell 2 | Cell 3

2+| Spans 2 columns | Cell 6

.2+| Spans 2 rows
| Cell 8
| Cell 9

| Cell 10
| Cell 11
|===

• 2+| = colspan of 2
• .2+| = rowspan of 2
• 2.3+| = colspan of 2, rowspan of 3

Current behavior

Spanning syntax is treated as literal content or causes parsing issues.

Implementation requirements

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

Add span fields to TableColumn:

pub struct TableColumn {
    pub content: Vec<Block>,
    pub colspan: usize,  // NEW: default 1
    pub rowspan: usize,  // NEW: default 1
}

2. Parser changes (acdc-parser/src/grammar/)

• Parse span prefixes before cell content: (\d+)?(\.\d+)?\+
• Validate spans don't exceed table boundaries
• Handle interaction with column specifiers

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

• Emit colspan and rowspan attributes on <td>/<th> elements
• Skip cells that are "covered" by a spanning cell from a previous row

Workaround

Use separate rows/columns instead of spanning.

Priority

High - affects complex table layouts, common use case.

References

• https://docs.asciidoctor.org/asciidoc/latest/tables/span-cells/