feat(auditlog): thread ParentExecutionId through S&F for retry-loop cached rows

The store-and-forward retry loop emits the per-attempt and terminal cached
audit rows (ApiCallCached/DbWriteCached Attempted, CachedResolve) via
CachedCallLifecycleBridge from a CachedCallAttemptContext, not from the
script context. The ExecutionId rollout (Task 4) already threaded ExecutionId
and SourceScript through this path; ParentExecutionId — the spawning
inbound-API request's ExecutionId — was not, so those retry-loop rows had
ParentExecutionId = null even for an inbound-API-routed run.

Thread it additively as a sibling at every carry point ExecutionId passes
through:

- StoreAndForwardMessage gains ParentExecutionId (Guid?).
- StoreAndForwardStorage adds a nullable parent_execution_id column via the
  same idempotent PRAGMA-probed ALTER TABLE migration; rows persisted by an
  older build read back null (back-compat). The defensive Guid.TryParse read
  helper (ParseExecutionId) is renamed ParseGuidColumn and reused for both
  columns so a corrupt value cannot abort the retry sweep.
- StoreAndForwardService.EnqueueAsync gains an optional parentExecutionId
  param, stamped onto the buffered message and surfaced on the
  CachedCallAttemptContext built in the retry loop.
- CachedCallAttemptContext gains ParentExecutionId.
- CachedCallLifecycleBridge.BuildPacket sets AuditEvent.ParentExecutionId
  from the context, beside the existing ExecutionId.
- IExternalSystemClient.CachedCallAsync / IDatabaseGateway.CachedWriteAsync
  gain an optional parentExecutionId param; ScriptRuntimeContext's CachedCall
  / CachedWrite helpers pass _parentExecutionId.

All threading is additive — ParentExecutionId is Guid? everywhere, null for
non-routed runs, and old buffered S&F rows still deserialize with the new
field null.

src/ScadaLink.ExternalSystemGateway/DatabaseGateway.cs

@@ -86,7 +86,8 @@ public class DatabaseGateway : IDatabaseGateway
         CancellationToken cancellationToken = default,
         TrackedOperationId? trackedOperationId = null,
         Guid? executionId = null,
-        string? sourceScript = null)
+        string? sourceScript = null,
+        Guid? parentExecutionId = null)
     {
         var definition = await ResolveConnectionAsync(connectionName, cancellationToken);
         if (definition == null)
@@ -132,7 +133,12 @@ public class DatabaseGateway : IDatabaseGateway
             // the retry-loop cached-write audit rows carry the same provenance
             // the script-side cached rows do.
             executionId: executionId,
-            sourceScript: sourceScript);
+            sourceScript: sourceScript,
+            // Audit Log #23 (ParentExecutionId Task 6): thread the spawning
+            // inbound-API request's ExecutionId onto the buffered row so the
+            // retry-loop cached-write audit rows correlate back to the
+            // cross-execution chain. Null for a non-routed run.
+            parentExecutionId: parentExecutionId);
     }
 
     /// <summary>

src/ScadaLink.ExternalSystemGateway/ExternalSystemClient.cs

@@ -88,7 +88,8 @@ public class ExternalSystemClient : IExternalSystemClient
         CancellationToken cancellationToken = default,
         TrackedOperationId? trackedOperationId = null,
         Guid? executionId = null,
-        string? sourceScript = null)
+        string? sourceScript = null,
+        Guid? parentExecutionId = null)
     {
         var (system, method) = await ResolveSystemAndMethodAsync(systemName, methodName, cancellationToken);
         if (system == null || method == null)
@@ -152,7 +153,12 @@ public class ExternalSystemClient : IExternalSystemClient
                 // buffered row so the retry-loop cached-call audit rows carry
                 // the same provenance the script-side cached rows do.
                 executionId: executionId,
-                sourceScript: sourceScript);
+                sourceScript: sourceScript,
+                // Audit Log #23 (ParentExecutionId Task 6): thread the spawning
+                // inbound-API request's ExecutionId onto the buffered row so
+                // the retry-loop cached-call audit rows correlate back to the
+                // cross-execution chain. Null for a non-routed run.
+                parentExecutionId: parentExecutionId);
 
             return new ExternalCallResult(true, null, null, WasBuffered: true);
         }