PingKit 0.5.0

PingKit 0.5.0 adds dual-stack IPv6 ping and a formal pingkit CLI, and
folds in the previously unreleased 0.4.0 work (outgoing TTL + .sent
events). This is a breaking minor: see below before upgrading.

Highlights

IPv6 ping (dual-stack)

• Full ICMPv6 echo over unprivileged datagram sockets, with hop-limit
  ancillary data and source address via recvmsg.
• PingConfiguration.AddressFamily — .automatic (follows the system's
  getaddrinfo ordering, so hostnames resolve over DNS64/NAT64), .ipv4,
  or .ipv6. IPv6 literals and scoped link-local addresses are supported.
• ICMPv6 Destination Unreachable, Time Exceeded, Packet Too Big, and
  Parameter Problem map to typed PingResponse events.
• On Darwin the socket sets ICMP6_FILTER so it only wakes for the
  message types ping consumes (not NDP/RA multicast).
• Tracer remains IPv4-only in this release.

Outgoing TTL / hop limit (was 0.4.0)

• PingConfiguration.timeToLive (1...255; nil keeps the system default),
  applied as IPv4 TTL or IPv6 unicast hop limit.
• CLI -m sets the outgoing TTL in ping mode.

.sent probe events (was 0.4.0)

• PingResponse.sent(sequence:) fires as each echo request leaves the
  socket, before that sequence's terminal event — so a UI can insert a
  pending row per probe and update it in place.

Formal CLI

• swift-argument-parser-based pingkit command with ping / trace
  subcommands, typed validation, and generated help. -4 / -6 force the
  address family. The dependency links only into the CLI target; the
  PingKit library stays zero-dependency.

Breaking changes

• PingReply.from is now IPAddress (was IPv4Endpoint).
• PingResponse gains .sent, .packetTooBig, and .parameterProblem
  cases — exhaustive switches must handle them.
• The executable product is named pingkit-cli (the command is still
  pingkit); the old ping-cli product is removed.

Notes

• unreachable(code:) codes are numbered differently by IPv4 vs IPv6;
  interpret code according to the address family the run resolved to.
• Verified on macOS, iOS Simulator (including ICMPv6 loopback integration
  tests), and Linux Swift 6.0 / 6.1.