Session 43: CLI config option and state machine design

Add command-line argument parsing with clap:
- -c/--config option to specify config file path
- Overrides default search path when provided
- Proper error handling for missing/invalid files

Improve logging:
- Peer connection log now uses separate log entries per field
- Better readability with aligned timestamps

Fix ICMPv6 error handling:
- Add multicast destination filter to should_send_icmp_error()
- Router Solicitation packets (ff02::2) now silently dropped
- Add test case for multicast destination

Add phase-based state machine design document:
- Document pattern where lifecycle phases use distinct structs in enum
- PeerSlot::Connecting(PeerConnection) -> PeerSlot::Active(ActivePeer)
- Benefits: type safety, memory efficiency, security
- Describes timeout handling and lookup table requirements

docs/design/README.md

@@ -18,3 +18,5 @@ Protocol design specifications and analysis for the Federated Interoperable Peer
 | [fips-architecture.md](fips-architecture.md)               | Software architecture: entities, state machines, transport abstractions, configuration  |
 | [fips-architecture-review.md](fips-architecture-review.md) | Architecture review issues and resolution status                                        |
 | [fips-tun-driver.md](fips-tun-driver.md)                   | TUN interface driver: reader/writer threads, ICMPv6, packet flow                        |
+| [fips-state-machines.md](fips-state-machines.md)           | Phase-based state machine pattern: peer lifecycle, transitions, timeout handling        |
+| [fips-protocol-flow.md](fips-protocol-flow.md)             | Protocol message flow: packet channel, event loop, dispatching                          |