Fix BOM rendering and line number styles

[chid]

Fixes #2123 and #1837.

Summary

• Strip UTF-8 BOM characters during line ingestion so they do not affect diff classification, syntax highlighting, or width calculations.
• Resolve built-in line-numbers after theme features so theme-provided line-numbers-* styles are preserved when --line-numbers is enabled.
• Add regression coverage for both behaviors.

Verification

• cargo test