docs: write-completion correlation configuration and semantics

docs/DesignDecisions.md

@@ -534,6 +534,48 @@ against the live MXAccess attribute set.
 - [Alarm Client Discovery — Subtag provider](./AlarmClientDiscovery.md)
 - [gRPC Contract — provider_status and degraded fields](./Grpc.md)
 
+## Secured-Write Completion Correlation
+
+MXAccess writes are fire-and-forget: the toolkit call returns before the
+Galaxy commit, and the per-item outcome only exists in the later
+`OnWriteComplete` COM callback. The original unary write reply therefore
+proved worker-side command acceptance only, forcing consumers (OtOpcUa's
+GalaxyDriver) to report every write as provisionally good.
+
+For `WriteSecured`/`WriteSecured2` the worker now holds the unary reply for a
+bounded window (`MxGateway:Worker:WriteCompletionWaitMilliseconds`, default
+1.5 s, `0` disables; conveyed to the worker via
+`MXGATEWAY_WORKER_WRITE_COMPLETION_WAIT_MS`) and copies the matching
+callback's status rows onto `MxCommandReply.statuses`. Key choices, argued in
+[the design doc](./plans/2026-08-09-write-completion-correlation-design.md):
+
+- **Pump-wait on the STA, not a parked reply.** The executor holds the STA
+  thread but pumps Windows messages each poll — the shipped ReadBulk pattern —
+  because commands serialize per session anyway, so freeing the STA during the
+  wait buys nothing and a parked reply would change the dispatcher/pipe
+  contracts.
+- **Version baseline before the COM call** closes the fast-completion edge: a
+  callback that dispatches while `WriteSecured` is still on the stack still
+  correlates.
+- **Timeout returns today's shape** (protocol OK, empty statuses):
+  unconfirmed is honest; a synthesized failure row would trigger consumer-side
+  write-revert logic on slow-but-successful commits. The 1.5 s default stays
+  inside OtOpcUa's 2 s Tier A write-resilience budget.
+- **Parity preserved.** `protocol_status`/`hresult` keep describing
+  acceptance; the MXAccess outcome (success or failure) rides only in
+  `statuses[0]`; the `OnWriteComplete` event still streams unchanged (nothing
+  swallowed, nothing synthesized).
+- **Scope: secured writes only.** Plain `Write`/`Write2` and bulk writes stay
+  fire-and-forget — the wait would add a device round-trip per write to
+  high-rate supervisory loops.
+- **Best-effort correlation.** The callback carries only
+  `(hItem, statuses)` — no transaction id — so concurrent writes to the same
+  item within the window can swap rows; benign for the serialized single-write
+  consumer contract.
+- **Client cancellation needs no special path**: a caller abandoning the RPC
+  mid-wait leaves the worker to finish its bounded wait and reply; the gateway
+  discards the reply, the session is never faulted.
+
 ## Later Revisit Items
 
 These are explicit post-v1 revisit items, not open blockers:

docs/GatewayConfiguration.md

@@ -114,6 +114,7 @@ launch CWD (SEC-01, SEC-33).
 | `MxGateway:Worker:StartupProbeRetryAttempts` | `3` | Number of retry attempts for transient worker startup probe failures before pipe connection and handshake continue. |
 | `MxGateway:Worker:StartupProbeRetryDelayMilliseconds` | `250` | Delay between transient startup probe retry attempts. |
 | `MxGateway:Worker:PipeConnectAttemptTimeoutMilliseconds` | `2000` | Per-attempt timeout used by the worker named-pipe connect retry path. The overall pipe connection still stays under the startup budget. |
+| `MxGateway:Worker:WriteCompletionWaitMilliseconds` | `1500` | Bounded wait the worker holds a `WriteSecured`/`WriteSecured2` reply for the matching MXAccess `OnWriteComplete` callback, so the reply's `statuses` carry the real commit outcome. `0` disables the wait (pure fire-and-forget replies). Must be `>= 0`. The gateway conveys the value to the worker via the `MXGATEWAY_WORKER_WRITE_COMPLETION_WAIT_MS` environment variable. Consumers that time their own writes must budget above this wait: OtOpcUa's GalaxyDriver wraps gateway writes in a 2 s Tier A resilience timeout, so a deployment raising this option past ~2000 must raise that driver `ResilienceConfig` write timeout in step or slow-but-successful commits surface as consumer-side failures. |
 | `MxGateway:Worker:ShutdownTimeoutSeconds` | `10` | Grace period for worker shutdown before the gateway treats shutdown as failed and may kill the worker process tree. |
 | `MxGateway:Worker:HeartbeatIntervalSeconds` | `5` | Worker heartbeat send interval and gateway heartbeat check cadence input. |
 | `MxGateway:Worker:HeartbeatGraceSeconds` | `15` | Maximum age of the last worker heartbeat before the gateway faults the worker. This must be greater than or equal to `HeartbeatIntervalSeconds`. |