test(domain): SubmitRegistration + OpenZaakWorker use cases (refs #6)

Failing application-layer tests over fake ports (IWorkflowClient, IAclClient,
IRegistrationStore):
- Submit persists an INGEDIEND registration and starts the registratie process,
  recording the instance id — and persists *before* starting, so the worker can
  correlate the OpenZaakAanmaken job back to its aggregate (ADR-0009).
- The worker opens a zaak via the ACL and attaches it; an unknown registration
  throws (job left for redelivery); a redelivered job is idempotent and opens no
  second zaak (§8.6).

Handlers are stubs (no persistence / no ACL call) so the tests compile and fail
on their assertions; the green commit implements them.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

services/domain/Big.Application/Big.Application.csproj

@@ -0,0 +1,15 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <!-- The application layer (CLAUDE.md §9): use cases over ports. Depends on Domain only;
+       Infrastructure implements the ports. -->
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\Big.Domain\Big.Domain.csproj" />
+  </ItemGroup>
+
+</Project>