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/ScadaLink.SiteCallAudit.csproj

@@ -0,0 +1,31 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <!-- SiteCallAuditActor is an Akka actor (central singleton in Bundle F); Akka is an explicit dependency. -->
+    <PackageReference Include="Akka" />
+    <PackageReference Include="Microsoft.Extensions.DependencyInjection.Abstractions" />
+    <PackageReference Include="Microsoft.Extensions.Logging.Abstractions" />
+    <PackageReference Include="Microsoft.Extensions.Options" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="../ScadaLink.Commons/ScadaLink.Commons.csproj" />
+    <!-- Site Call Audit (#22) sits alongside Notification Outbox (#21) and Audit Log (#23).
+         ISiteCallAuditRepository is registered by ScadaLink.ConfigurationDatabase; the
+         project reference is documented here so the actor's scope-per-message
+         GetRequiredService<ISiteCallAuditRepository>() compiles. -->
+    <ProjectReference Include="../ScadaLink.ConfigurationDatabase/ScadaLink.ConfigurationDatabase.csproj" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <InternalsVisibleTo Include="ScadaLink.SiteCallAudit.Tests" />
+  </ItemGroup>
+
+</Project>