mbproxy: add keepalive / connection monitoring

The DL205/DL260 ECOM emits no TCP keepalives, so an idle backend socket
can be silently dropped by a middlebox (switch, firewall, NAT) after
2-5 minutes. Enable OS SO_KEEPALIVE on backend and accepted upstream
sockets, and drive a periodic synthetic FC03 heartbeat on each idle
backend socket so a dead path is detected before a real client request
hits it. Controlled by Connection.Keepalive (ON by default).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

mbproxy/docs/Architecture/ConnectionModel.md

@@ -240,6 +240,7 @@ The per-request timeout watchdog described above is the production defence again
 - [`./Overview.md`](./Overview.md) — proxy architecture entry point
 - [`./ReadCoalescing.md`](./ReadCoalescing.md) — FC03/FC04 fan-out built on `InterestedParties`
 - [`./ResponseCache.md`](./ResponseCache.md) — per-PLC FC03/FC04 cache layered in front of this multiplexer
+- [`./Keepalive.md`](./Keepalive.md) — TCP keepalive and the backend heartbeat that keeps this socket warm
 - [`../Operations/Configuration.md`](../Operations/Configuration.md) — `Connection.BackendConnectTimeoutMs`, `Connection.BackendRequestTimeoutMs`, retry tuning
 - [`../Operations/StatusPage.md`](../Operations/StatusPage.md) — `inFlight`, `maxInFlight`, `txIdWraps`, `queueDepth`, `disconnectCascades` counters
 - [`../Reference/LogEvents.md`](../Reference/LogEvents.md) — `mbproxy.multiplex.*` structured log events