Fix pktinfo send cmsg_level/cmsg_type (IPPROTO_IP / IP_PKTINFO)

[ianegordon]

Problem

The send-side IP_PKTINFO/IPV6_PKTINFO control messages built by Socket.withPacketInfoControl set:

header.pointee.cmsg_level = SOL_SOCKET
header.pointee.cmsg_type  = Socket.ipproto_ip   // protocol number in the type field

Per cmsg(3), cmsg_level is the originating protocol and cmsg_type the protocol-specific type. A pktinfo ancillary message requires IPPROTO_IP / IP_PKTINFO (RFC 3542 §6 for IPv6: "the socket option and cmsghdr level will be IPPROTO_IPV6, the type will be IPV6_PKTINFO"). The library's own receive path (getPacketInfoControl) already matches on exactly those pairs — so send and receive were mutually inconsistent, and send(message:to:interfaceIndex:from:) could not apply source-address/interface selection.

Change

Set cmsg_level = IPPROTO_IP / cmsg_type = IP_PKTINFO (IPv6: IPPROTO_IPV6 / IPV6_PKTINFO) on the send side, matching the receive path. Depends on the ipv6_pktinfo constant fix from #233 (already on main): with the correct cmsg level the kernel interprets cmsg_type for the first time, so both fixes are needed together for IPv6 sends to succeed on Darwin.

Tests

• Deterministic unit tests asserting the exact level/type pairs against the platform constants for IPv4 and IPv6 (verified to fail against the previous values).
• Loopback round-trip tests: datagrams sent with explicit packet info are received with the expected payload for both IPv4 and IPv6.

Full suite passes (464 tests).

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.72%. Comparing base (71dcb81) to head (8337bfc).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #238      +/-   ##
==========================================
+ Coverage   93.00%   93.72%   +0.71%     
==========================================
  Files          72       72              
  Lines        3748     3777      +29     
==========================================
+ Hits         3486     3540      +54     
+ Misses        262      237      -25     

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[swhitty]

nice, thank you