fix: honor explicit limits in text style

[techwizrd]

Summary

• Fixes Explicit \limits is ignored in inline math #67 by honoring explicit \limits in inline/text-style math.
• Preserves explicit \limits / \nolimits parser metadata so layout can distinguish user-requested limits from default operator behavior.
• Adds parser, layout, and golden KaTeX-vs-RaTeX regression coverage for text-style \sum limits.

Testing

• cargo build --workspace
• cargo clippy --workspace -- -D warnings
• cargo test --workspace
• cargo test -p ratex-render golden_test_pass_rate
• cargo test -p ratex-svg --features standalone --test golden_svg golden_svg_main_pass_rate

Golden scores for new cases:

• 0997 \textstyle\sum_{n=1}^{\infty}: 0.750
• 0998 \textstyle\sum\limits_{n=1}^{\infty}: 0.702

[erweixin]

Looks great to me. Thanks for your help!