Fixes #24

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• I have checked that there isn't already another pull request that solves the above issue
• All newly added code is adequately covered by tests
• All existing tests are still running without errors

AI Disclosure: GitHub Copilot (Claude Opus 4.6) was used to assist with implementation, code review, and unit test creation. All changes were reviewed and validated by me. Copilot was used mainly to generate unit tests.

Changes

Adds column spanning support for table body rows, as requested in #24.

New API

A new TableCell class wraps cell content with a fluent .Span(int) API:

var table = new Table();
table.AddColumns("Item", "Qty", "Unit Price", "Total");
table.AddRow("Item A", "5", "20", "100");
table.AddRow(new IRenderable[] {
    new Markup("Item B"),
    new TableCell("Note: deliver after 5pm").Span(3)
});
table.AddRow("Item C", "3", "15", "45");

Scope

• Column spanning only — row spanning is left for future work
• Body rows only — spanning in headers/footers throws InvalidOperationException
• Row separator lines (├──┼──┤) don't yet merge under spanned cells (cosmetic, can be addressed in a follow-up)

Files changed

• New: TableCell.cs — IRenderable wrapper with ColumnSpan property and fluent Span() method
• Modified: TableRenderer.cs — span-aware cell width calculation, correct separator/edge handling for spanned cells
• Modified: TableMeasurer.cs — distributes spanned cell measurements proportionally across columns
• Modified: TableRowCollection.cs — span-aware row validation (total span vs column count)
• Modified: TableColumn.cs — rejects TableCell with span > 1 in Header/Footer property setters
• Modified: TableRow.cs — rejects spanning cells in header/footer row constructors

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