Add bind_address config option for local IP binding

[kofany]

Summary

• Adds a bind_address configuration option that allows clients to bind to a specific local IP address when establishing a connection to an IRC server
• Useful for multi-homed machines, vhost setups, and running multiple bots from different IP addresses on the same host
• Uses tokio::net::TcpSocket to bind before connecting, with proper IPv4/IPv6 address family matching during DNS resolution

Changes

• src/client/data/config.rs: New bind_address: Option<String> field with serde support and bind_address() getter
• src/client/conn.rs: New connect_with_bind() helper using TcpSocket::bind() + connect(), integrated into both proxy and non-proxy new_stream() variants
• src/error.rs: New ConfigError::InvalidBindAddress variant for invalid IP address strings

Usage

TOML config

nickname = "mybot"
server = "irc.example.com"
bind_address = "192.168.1.100"

Programmatic

let config = Config {
    nickname: Some("mybot".to_owned()),
    server: Some("irc.example.com".to_owned()),
    bind_address: Some("192.168.1.100".to_owned()),
    ..Config::default()
};

Notes

• When bind_address is None (default), behavior is unchanged — the OS selects the local address
• IPv4/IPv6 family of the bind address is matched against resolved server addresses
• SOCKS5 proxy: a warning is logged since the proxy library manages its own TCP connection
• No new dependencies required (TcpSocket is already available via tokio's net feature)

Test plan

• cargo check — compiles cleanly
• cargo clippy --all-targets --all-features — no new warnings
• cargo test — all 67 unit tests + 8 doc-tests pass
• Manual testing with a multi-homed machine

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