Phase 8: Production readiness — failover tests, security hardening, sandboxing, deployment docs

- WP-1-3: Central/site failover + dual-node recovery tests (17 tests)
- WP-4: Performance testing framework for target scale (7 tests)
- WP-5: Security hardening (LDAPS, JWT key length, no secrets in logs) (11 tests)
- WP-6: Script sandboxing adversarial tests (28 tests, all forbidden APIs)
- WP-7: Recovery drill test scaffolds (5 tests)
- WP-8: Observability validation (structured logs, correlation IDs, metrics) (6 tests)
- WP-9: Message contract compatibility (forward/backward compat) (18 tests)
- WP-10: Deployment packaging (installation guide, production checklist, topology)
- WP-11: Operational runbooks (failover, troubleshooting, maintenance)
92 new tests, all passing. Zero warnings.

src/ScadaLink.Commons/Interfaces/Services/IExternalSystemClient.cs

@@ -0,0 +1,37 @@
+namespace ScadaLink.Commons.Interfaces.Services;
+
+/// <summary>
+/// Interface for invoking external system HTTP APIs.
+/// Implemented by ExternalSystemGateway, consumed by ScriptRuntimeContext.
+/// </summary>
+public interface IExternalSystemClient
+{
+    /// <summary>
+    /// Synchronous call to an external system. All failures returned to caller.
+    /// </summary>
+    Task<ExternalCallResult> CallAsync(
+        string systemName,
+        string methodName,
+        IReadOnlyDictionary<string, object?>? parameters = null,
+        CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Attempt immediate delivery; on transient failure, hand to S&amp;F engine.
+    /// Permanent failures returned to caller.
+    /// </summary>
+    Task<ExternalCallResult> CachedCallAsync(
+        string systemName,
+        string methodName,
+        IReadOnlyDictionary<string, object?>? parameters = null,
+        string? originInstanceName = null,
+        CancellationToken cancellationToken = default);
+}
+
+/// <summary>
+/// Result of an external system call.
+/// </summary>
+public record ExternalCallResult(
+    bool Success,
+    string? ResponseJson,
+    string? ErrorMessage,
+    bool WasBuffered = false);