feat: support multicolumn table cells

[Happypig375]

Summary

• add table-cell span, alignment, and column-spec metadata for \multicolumn
• parse and serialize canonical multicolumn syntax while preserving table cloning, equality, and public API behavior
• typeset spanned widths and effective vertical rules across ordinary and delimiter-wrapped matrices
• cover ragged/following cells, vertical rules, hlines, delimiters, malformed spans, and matrix contracts

Testing

• MulticolumnTests: 8 passed
• Core test project Release build passed with 0 warnings/errors
• scoped format verification and git diff --check passed

Stack

This PR is based on port-iosmath-2026 / #264 because the table model and parser work build on that port.

Closes #304