Refine Communication Layer: timeouts, transport config, ordering, failure behavior

Add per-pattern message timeouts with sensible defaults (120s for deployments, 30s
for queries/commands). Configure Akka.NET transport heartbeat explicitly rather than
relying on framework defaults. Document per-site message ordering guarantee. Specify
that in-flight messages on disconnect result in timeout error (no central buffering)
and debug streams die on any disconnect.

docs/plans/2026-03-16-communication-layer-refinement-design.md

@@ -0,0 +1,47 @@
+# Communication Layer Refinement — Design
+
+**Date**: 2026-03-16
+**Component**: Central–Site Communication (`Component-Communication.md`)
+**Status**: Approved
+
+## Problem
+
+The Communication Layer doc defined 8 message patterns clearly but lacked specification for timeouts, transport configuration, reconnection behavior, message ordering guarantees, and connection failure handling.
+
+## Decisions
+
+### Message Timeouts
+- **Per-pattern timeouts with sensible defaults**, overridable in configuration.
+- Deployment and system-wide artifacts: 120 seconds (script compilation can be slow).
+- Lifecycle commands, integration routing, recipe/command delivery, remote queries: 30 seconds.
+- Uses the Akka.NET ask pattern; timeout results in failure to caller.
+
+### Transport Configuration
+- **Akka.NET built-in reconnection** with explicitly configured transport heartbeat interval and failure detection threshold.
+- No custom reconnection logic — framework handles it.
+- Settings explicitly documented rather than relying on framework defaults, for predictability in a SCADA context.
+
+### Connection Failure Behavior
+- **In-flight messages get a timeout error** — caller retries manually. No buffering at central. Consistent with existing design principle.
+- Automatic retry rejected due to risk of duplicate processing (e.g., site may have applied a deployment before the connection dropped).
+
+### Message Ordering
+- **Per-site ordering guaranteed** — relies on Akka.NET's built-in per-sender/per-receiver ordering. No custom sequencing logic needed.
+
+### Debug Stream Interruption
+- **Stream dies on any disconnect** (failover or network blip). Engineer reopens the debug view manually.
+- Auto-resume rejected — adds complexity for a transient diagnostic tool.
+
+## Affected Documents
+
+| Document | Change |
+|----------|--------|
+| `Component-Communication.md` | Added 4 new sections: Message Timeouts, Transport Configuration, Message Ordering, Connection Failure Behavior |
+
+## Alternatives Considered
+
+- **Global timeout for all patterns**: Rejected — deployment involves compilation and needs more time than a simple query.
+- **Default Akka.NET transport settings**: Rejected — relying on undocumented defaults is risky for SCADA; explicit configuration ensures predictable behavior.
+- **Automatic retry of in-flight messages**: Rejected — risks duplicate processing and contradicts the no-buffering-at-central principle.
+- **No ordering guarantee**: Rejected — Akka.NET provides this for free; the design already implicitly relies on it.
+- **Auto-resume debug streams on reconnection**: Rejected — adds state tracking complexity for a transient diagnostic feature.