feat(scaudit): SiteCallAuditActor minimum surface (#22, #23 M3)

Bundle C of Audit Log #23 M3.

Adds the ScadaLink.SiteCallAudit project + matching tests project, mirroring
the ScadaLink.AuditLog scaffolding pattern (net10.0, central package
management, InternalsVisibleTo to the tests assembly).

SiteCallAuditActor is the central singleton entry point for Site Call Audit
(#22): it receives UpsertSiteCallCommand and persists the SiteCall via
ISiteCallAuditRepository.UpsertAsync (monotonic, idempotent — out-of-order
or duplicate updates are silent no-ops at the repo). Audit-write failures
NEVER abort the user-facing action (CLAUDE.md): repository throws are
caught + logged, the actor replies Accepted=false, and the singleton stays
alive (Resume supervisor strategy as defence in depth).

Two constructors mirror AuditLogIngestActor:
- IServiceProvider production constructor resolves the scoped EF repository
  from a fresh DI scope per message.
- ISiteCallAuditRepository test constructor injects a concrete repository so
  the TestKit tests exercise the real monotonic-upsert SQL end to end.

UpsertSiteCallCommand + UpsertSiteCallReply live in ScadaLink.Commons (same
home as IngestAuditEventsCommand) so Bundle D's gRPC server can construct
them without taking a project reference on the actor's host project.

AddSiteCallAudit() is a placeholder for symmetry with AddAuditLog /
AddNotificationOutbox; Bundle F will populate it with the actor's Props
factory + options bindings.

Tests (Akka.TestKit.Xunit2 + MsSqlMigrationFixture via project ref to
ScadaLink.ConfigurationDatabase.Tests, mirroring Bundle D2):
- Receive_UpsertSiteCallCommand_Persists_Replies_Accepted
- Receive_DuplicateUpsert_OlderStatus_NoOp_StillRepliesAccepted (idempotency)
- Receive_RepoThrowsTransient_RepliesAccepted_False_ActorStaysAlive

Reconciliation, KPIs, and the central->site Retry/Discard relay are
deferred per CLAUDE.md scope discipline.

ScadaLink.slnx updated to include both new projects.

All 3 new tests pass against the running infra/mssql container; full suite
(2683 tests across 27 projects) passes with no regressions.

src/ScadaLink.SiteCallAudit/ServiceCollectionExtensions.cs

@@ -0,0 +1,39 @@
+using Microsoft.Extensions.DependencyInjection;
+
+namespace ScadaLink.SiteCallAudit;
+
+/// <summary>
+/// Composition root for the Site Call Audit (#22) component.
+/// </summary>
+/// <remarks>
+/// <para>
+/// M3 Bundle C ships the ingest-only minimum surface (the actor itself); the
+/// full DI surface — reconciliation puller, KPI projector, central→site
+/// Retry/Discard relay, options + validators — is deferred to a follow-up.
+/// </para>
+/// <para>
+/// The repository (<c>ISiteCallAuditRepository</c>) is registered by
+/// <c>ScadaLink.ConfigurationDatabase.ServiceCollectionExtensions.AddConfigurationDatabase</c>,
+/// so callers (the Host on the central node) must also call that. The actor's
+/// <c>Props</c> are wired up in Host registration (Bundle F); this extension
+/// is currently a no-op placeholder kept for symmetry with the AuditLog and
+/// NotificationOutbox composition roots — adding it now means consumers can
+/// reference the method without re-touching the Host project later.
+/// </para>
+/// </remarks>
+public static class ServiceCollectionExtensions
+{
+    /// <summary>
+    /// Registers Site Call Audit (#22) services. Currently a no-op
+    /// placeholder — Bundle F will populate this with the actor's Props
+    /// factory + options bindings. The method is exposed now so the Host
+    /// wiring call already exists at the API boundary.
+    /// </summary>
+    public static IServiceCollection AddSiteCallAudit(this IServiceCollection services)
+    {
+        ArgumentNullException.ThrowIfNull(services);
+        // Actor props are constructed in Host wiring (Bundle F). This
+        // extension is a placeholder for future config + DI.
+        return services;
+    }
+}