Add --multicast-ttl CLI flag for forwarded packets

[schmiatz]

Summary

• Adds a --multicast-ttl CLI option (also settable via MULTICAST_TTL env var) to control the IP multicast TTL (hop limit) on outbound forwarded packets
• When forwarding to multicast destinations, the OS default TTL is 1, which prevents packets from being routed beyond the local network segment
• This flag allows operators to set a higher TTL (e.g. --multicast-ttl 64) so multicast traffic can traverse routers

Changes

• proxy/src/main.rs: Added multicast_ttl: Option<u32> to CommonArgs, passed to forwarder
• proxy/src/forwarder.rs: Calls set_multicast_ttl_v4() on send sockets when the option is set
• proxy/src/forwarder.rs: Added unit test for TTL behavior

Usage

jito-shredstream-proxy forward-only \
  --multicast-ttl 64 \
  --dest-ip-ports 233.84.178.11:7744,10.0.0.1:8888