docs(protocol): airlock server black-box findings

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Transfer independently black-box-verified server behavior from a source-assisted
research phase conducted outside this repository. Only a sanitizer-candidate
report of black-box findings and the packet captures it referenced were admitted;
no upstream source, tests, history, harnesses, build trees, result files or
source-assisted analysis crossed the boundary. Provenance and the boundary
discipline are recorded in docs/clean-room/CLEANROOM_NOTES.md.

New upstream robustness defect (Section 22.5). An open selecting timestamps while
omitting the Clock parameter is accepted with a valid token and no Clock echoed,
and the first echo on that session is fatal to the server process, taking
unrelated sessions on the same process with it. An explicit Clock=0 is instead
rejected at open, so parameter presence is externally meaningful. Recorded as an
observed defect with a must-not-crash recommendation; the clean server's
resolution is deliberately left to the implementation work.

Also established or strengthened:

- the idle grace and maximum-duration grace are fixed additive margins across the
  settings swept, and the maximum-duration clock starts at the first echo that is
  actually served;
- a rate-limited request is the only tested drop class that keeps a session
  alive, and in one documented configuration a conforming client can observe the
  difference;
- a post-deadline packet of several tested classes consumes an expired session's
  one remaining reply, including packets that are themselves dropped;
- the reply-length cap is the bind interface's MTU on the interfaces where it was
  reachable, and a length the host cannot transmit ends the process;
- DSCP marking follows the session, with unmarked open replies and no
  cross-session leakage across interleaved sessions;
- repeated tags are last-valid-wins, with invalid-duplicate handling scoped to
  the orders actually tested, and unknown tag values are consumed as varints.

Three captures were admitted, each rewritten to the container convention already
used here and verified byte-identical in packet data and timestamps:
server-clock-absent (3 packets), server-expiry-consumption (14) and
server-dscp-interleaved (32). Two further captures were reviewed and rejected as
redundant or unsupporting; their textual results were kept instead.

Host-specific figures are labelled as such throughout, upstream defects are not
promoted to protocol requirements, observations are separated from inferences and
from recommendations, and no implementation policy is chosen here. The client
specification gains matching refinements where the two compatibility baselines
would otherwise state a lifecycle or marking rule differently.

Documentation and evidence only; no production code changed.

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