Clarify end_column semantics with detailed comments
Added comprehensive documentation explaining the inclusive vs exclusive
semantics of end_column:
**Single-line errors:**
- After normalization, end_column is EXCLUSIVE (one past last char)
- Example: start=5, end=6 marks exactly column 5
- This matches Rust's range semantics [start, end)
**Multiline errors:**
- User provides end_column as INCLUSIVE (the last column to mark)
- Example: end_column=12 means mark UP TO AND INCLUDING column 12
- We convert to exclusive internally with +1 for consistent calculation
**Why the difference?**
- Single-line: Matches typical API expectations (like string slicing)
- Multiline: More intuitive for users to specify "mark to column 12"
- Internal calc uses exclusive ranges for consistency
This explains the seemingly inconsistent +1 logic in calculate_marker_position.
All 20 tests passing.
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>
