fix: Typst codegen bugs for math markup and edge cases

[developer0hye]

Summary

• US-310: Add groupChr element handling — maps OMML ⏞ (overbrace) and ⏟ (underbrace) to Typst overbrace() / underbrace() functions
• US-311: Subscript/superscript syntax already correctly wraps multi-token content in parentheses via wrap_if_needed(); added regression tests
• US-312: Fix empty radicand in sqrt() / root() — emit placeholder "" instead of missing argument to prevent Typst "missing argument: radicand" error
• US-313: Fix delimiter balancing — when begChr or endChr is empty, omit both delimiters to prevent unmatched bracket errors in Typst
• US-314: Wrap non-ASCII alphabetic text (Cyrillic, CJK, etc.) in math mode with upright("...") to prevent Typst "unknown variable" errors
• US-315: Escape [, ], {, } in text content alongside existing Typst special character escaping to prevent premature content block closure
• US-316: Fall back to solid fill when gradient has < 2 stops — prevents Typst "a gradient must have at least two stops" error

Related: #80, #81

Test plan

• Unit tests for all 7 user stories (groupChr, sub/superscript, empty radicand, delimiter balancing, non-ASCII math text, text escaping, single-stop gradient)
• cargo test --workspace passes (722+ tests)
• cargo fmt --all -- --check passes
• cargo clippy --workspace -- -D warnings passes
• CI passes

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