Fix overlapping entity conflict resolution (#33)

Fix overlapping entity conflict resolution (#33)

Implement Presidio-style conflict resolution for overlapping PII entities.

Problem: Presidio returns overlapping entities (e.g., both "Eric" and "Eric's")
which caused text corruption during masking.

Solution: Two-phase algorithm matching Presidio's Anonymizer logic:
1. Merge overlapping entities of same type (expand boundaries, keep highest score)
2. Remove conflicting entities of different types (contained or lower score loses)

- Add conflict-resolver.ts with resolveConflicts() and resolveConflictsSimple()
- Replace old removeOverlappingEntities with new conflict resolver
- Add 18 tests covering all conflict scenarios
- Delete old entities.ts (replaced by conflict-resolver.ts)