feat: add cell vertical alignment in tables

[developer0hye]

Summary

• add CellVerticalAlign (Top/Center/Bottom) and vertical_align to the public TableCell IR
• parse DOCX table cell vAlign and render Typst table.cell(align: ...) overrides
• preserve exact DOCX row heights and emit Typst rows: (...) sizing so vertical alignment takes effect in fixed-height rows

Why

• DOCX table cell vertical alignment was ignored
• row-height preservation was needed for center/bottom alignment to have visible effect in common Word tables

Key changes

• parse verticalAlign from DOCX cell properties
• carry exact DOCX row heights (trHeight with hRule=exact) into TableRow.height
• emit Typst table row sizing when any row has an explicit height
• add parser and renderer coverage for vertical alignment and row-height behavior

Compatibility

• TableCell is public through office2pdf::ir, so adding vertical_align is a public API change for downstream struct literals/destructuring

Test plan

• cargo test -p office2pdf table --lib --offline
• cargo test -p office2pdf row_height --lib --offline

Related: #111