feat: enhance file history range tracking with comprehensive overlap handling

Improved calculateWriteRanges to properly detect modified content ranges by
comparing original and modified content line-by-line instead of marking
entire files as valid.

- Added computeModifiedRanges function for precise change detection
- Added adjustHistoricalRanges for proper range adjustment after modifications
- Added comprehensive test coverage for all overlap scenarios
- Tests validate range splitting, merging, and shift calculations
- Handles partial overlaps, insertions, deletions, and edge cases

The range tracking now accurately preserves historical valid ranges while
only invalidating ranges that were actually modified.

Fixes #5871 #1374

Signed-off-by: Eric Wheeler <[email protected]>

Author: Eric Wheeler