Phase 4: Add syntax highlighting tests and fix BytePos offset
Added comprehensive tests for the highlighting module:
- test_extract_highlights_basic: Verify single-line highlighting
- test_extract_highlights_multiline: Verify multi-line handling
- test_apply_line_highlights_basic: Verify ANSI code insertion
- test_apply_line_highlights_plain: Verify plain mode
Fixed critical bug: BytePos in SWC is 1-indexed (BytePos(0) is reserved),
so we need to subtract 1 to get 0-indexed offsets for our markers.
All 25 tests now pass (21 original + 4 highlighting tests).
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>
