Move wire connectivity logic from _handle_get_wire_connections into
commands/wire_connectivity.py. Use KiCad's internal integer unit system
(10,000 IU/mm) with exact coordinate matching instead of tolerance-based
float comparison, mirroring how KiCad itself determines connectivity.
Key improvements:
- Exact integer matching for wire endpoints (O(1) dict lookup vs O(n) grid scan)
- Junction support for T-connections
- Multi-unit symbol support (removed incorrect processed_refs dedup)
- Single public API: get_wire_connections()
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
