Implemented connection handshake protocol for federation building on
the Instance Identity Model from issue #84.
**Services:**
- SignatureService: Message signing/verification with RSA-SHA256
- ConnectionService: Federation connection management
**API Endpoints:**
- POST /api/v1/federation/connections/initiate
- POST /api/v1/federation/connections/:id/accept
- POST /api/v1/federation/connections/:id/reject
- POST /api/v1/federation/connections/:id/disconnect
- GET /api/v1/federation/connections
- GET /api/v1/federation/connections/:id
- POST /api/v1/federation/incoming/connect
**Tests:** 70 tests pass (18 Signature + 20 Connection + 13 Controller + 19 existing)
**Coverage:** 100% on new code
**TDD Approach:** Tests written before implementation
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
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