Phase 2: Multicast receive (cherry-pick from stephram/oscpack)

[tap]

Part of Phase 2 — Reach (ROADMAP.md).

Add multicast UDP receive, cherry-picking from the stephram/oscpack fork.

• Port the multicast group join/leave (IP_ADD_MEMBERSHIP) to the ip/posix and ip/win32 UdpSocket backends.
• Public API to bind a multicast group + interface; mirror the existing socket types.
• Keep the oscpack:: alias on any new types (compat moat).
• Tests on the POSIX matrix.

Self-contained and additive — the next demand-driven feature pickup after the freestanding groundwork. (Note: the uncompiled ip/*/UdpSocket.h backends still carry the strcpy/gethostbyname cleanup deferred from #4 — fold that in when these enter the compiled CI surface.)