The gateway half of the going-idle/buffered-flip primitive (scale-to-zero
PRIMITIVE, not the behaviour). Integrates with the EXISTING drain transition:
- ws_transport: `go_idle()` sends `going_idle` + awaits the connector's
`going_idle_ack` (connector-authoritative flip-then-ack, Q-5.3c — stays
serving until the ack so nothing is lost in the flip window); acks a buffered
inbound (bufferId present) via `inbound_ack` after the handler runs
(drain-without-dup on the delivery leg); NET-NEW reconnect loop re-dials +
re-handshakes after an unexpected close (off by default, on in production).
- adapter: emits `going_idle` from its existing `disconnect()` drain seam before
tearing down the socket; best-effort + guarded (never blocks shutdown).
- transport Protocol + contract doc §3.2 document the 3 new frames.
+6 relay tests (124 pass). NOT in scope: the autonomous idle timer / machine
suspend / NAS health model (deferred behaviour). Ben's relay-adapter solo lane.
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