Declare `_TERMINAL_HEALTH_REASONS = frozenset({"socket_closed",
"client_closed"})` beside `_read_websocket_health` (the producer of those
literals at adapter.py:1705/:1707/:1715) and use it at the escalation
site in `_liveness_loop` (was `reason in ("socket_closed",
"client_closed")` at :1791).
WHY: the first-strike classification was a magic-string match against
literals produced 80 lines away with no shared definition. A rename in
the producer, or a new hard-closed reason, would silently demote the
escalation back to the threshold path with nothing failing. One named
set next to the producer makes the coupling visible. The
`tuple[bool, str]` return shape is unchanged (existing tests monkeypatch
the sampler with 2-tuple lambdas).
Proof: mutating the set to {"socket_closed"} fails
test_closed_transport_first_strike_forces_reconnect[client_closed];
restored, the liveness file is green.
c2d7bb89f1