- Add TUN interface detection on startup, delete existing before create
- Add proper interface deletion on shutdown via netlink
- Add TUN packet reader in separate thread with DEBUG logging
- Add graceful shutdown handling for expected "Bad address" error
- Add take_tun_device() to Node for reader ownership transfer
- Add shutdown_tun_interface() public function for cleanup by name
- Add tokio signal feature for Ctrl+C handling
Implements FIPS Identity System (Section 1 of design doc):
- NodeId: 32-byte SHA-256 hash of npub, with Ord for root election
- FipsAddress: 128-bit IPv6-compatible address (0xfd prefix)
- Identity: keypair holder with sign/verify methods
- AuthChallenge/AuthResponse: challenge-response authentication
All 11 tests passing.