PingKit 0.6.0

PingKit 0.6.0 makes send failures non-fatal. A probe the socket rejects
(e.g. an IPv4 literal forced onto an IPv6-only network, or a briefly
unreachable interface) no longer aborts the whole run — it surfaces as a
.sendFailed event and the session recovers on the next interval.

This is a breaking minor: PingResponse gains a case.

New invariant

Each sequence produces exactly one of two outcomes:

• (a) .sent followed by a terminal event (.reply, .timeout,
  .unreachable, .timeExceeded, .packetTooBig, .parameterProblem) —
  the socket accepted the probe; or
• (b) a single .sendFailed(sequence:errno:) with no .sent — the
  probe never left the socket.

Changes

• PingResponse.sendFailed(sequence: UInt16, errno: Int32) — new case.
• Non-fatal send path: Pinger no longer finishes the stream on a send
  error. The failed probe isn't counted in transmitted, gets no pending
  entry and no .sent; the run continues and recovers when the network does.
  A bounded .times(n) run still terminates (the failed slot completes).
• One-shot ping(_:) still throws PingError.sendFailed(errno:).
• Statistics: failed sends are not counted toward transmitted.
• CLI: prints Send failed for icmp_seq N (errno …) per failure and
  exits non-zero when nothing came back — whether probes were sent and lost
  or never left the socket at all — closer to ping(8).

Breaking change

PingResponse gains .sendFailed; exhaustive switches must handle it.

Notes

Verified on macOS, iOS Simulator, and Linux Swift 6.0 / 6.1 (72 tests).
End-to-end: pingkit-cli 8.8.8.8 -6 (IPv4 literal forced to IPv6, errno 22)
now reports a send failure per probe and runs to completion instead of
aborting.