Fix overlapping entity masking (#33)

Fix overlapping entity masking (#33)

Presidio can return overlapping PII detections (e.g., both "Eric" and
"Eric's" as separate PERSON entities). When both were masked, the string
positions became invalid, corrupting the output.

Added removeOverlappingEntities() to filter overlaps before masking:
- Sort by start position (longer first if same start)
- Keep only non-overlapping entities