On Linux, best-effort upgrade the socket's SO_TIMESTAMPING flags to
TX_SOFTWARE | OPT_ID | OPT_TSONLY after a successful Open and before the
first Echo probe. Promote the #103 error-queue foundation to production:
MSG_ERRQUEUE is drained opportunistically with a fixed per-call budget at
several natural choke points (after a probe send, before processing a
reply, before evaluating timeouts), and each automatic OPT_ID timestamp is
correlated directly with wire_seq — safe because Linux rolls back its
speculative sk_tskey increment on every send failure, wire_seq and the
kernel counter start together at every Open, and a socket can never be
reopened for a second session. The observed kernel TX wall timestamp is
retained on PendingProbe (surviving pending -> timed-out retention) as
dormant metadata only; RTT, OWD, and IPDV are unchanged and continue to use
the userspace send timestamp exclusively. Capability failure is fully
non-fatal and RX timestamping is unaffected either way. Non-Linux and
non-ancillary builds are unchanged.
Nils Andreas Svee