feat: add tab stop parsing and improved tab rendering

[developer0hye]

Summary

• add DOCX tab stop support end to end through IR, parsing, and Typst generation
• preserve inherited style tab stops while applying paragraph-level overrides, including clear semantics for targeted tab positions
• render explicit tab leaders and improve decimal-tab alignment for values with thousands separators and locale-specific decimal separators

Why

Word tab stops are inherited through styles, can be selectively cleared, and often depend on dotted leaders or decimal alignment. The initial implementation covered the basics but still replaced inherited stops wholesale and aligned decimal tabs on the first punctuation mark.

Key changes

• add tab stop override resolution in the DOCX parser so paragraph-level tabs merge correctly with inherited style tabs
• emit leader-filled tab gaps in Typst instead of plain spacing when the tab stop defines a leader
• anchor decimal tabs on the actual decimal separator instead of the first . or ,
• add parser, Typst generation, and render-path regression tests for tab inheritance, clear behavior, leaders, and decimal alignment

Test plan

• cargo test -p office2pdf
• GitHub Actions checks
• Visual verification with DOCX containing tabbed content

Related: #111