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

Cargo.toml

@@ -13,6 +13,7 @@ serde = { version = "1.0", features = ["derive"] }
 serde_yaml = "0.9"
 dirs = "6.0"
 hex = "0.4"
+clap = { version = "4.5", features = ["derive"] }
 tracing = "0.1"
 tracing-subscriber = { version = "0.3", features = ["env-filter"] }
 tun = { version = "0.7", features = ["async"] }