fix(acl): keep the integration tests out of Stryker's solution (refs #174)

Stryker runs every test project in Acl.slnx regardless of test-projects, so
Acl.IntegrationTests (needs a live OpenZaak) failed 8 tests in every initial
run. Acl.slnx is read only by Stryker; the root solution still builds it.
Re-measured with a green initial run: 90.45 %, so break: 90 stands.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/runbooks/ci.md

@@ -60,6 +60,9 @@ make mutation     # dotnet tool restore + dotnet stryker on the ACL
 Config lives in [`services/acl/stryker-config.json`](../../services/acl/stryker-config.json).
 It runs in **solution mode** against `Acl.slnx`, mutating the two projects under test
 (`Acl.Application`, `Acl.Infrastructure`); `Acl.Api` has no tests and is skipped.
+`Acl.slnx` leaves out `Acl.IntegrationTests`: it needs a live OpenZaak, and Stryker
+runs every test project in the solution, so keeping it in makes 8 tests fail in the
+initial run (#174).
 
 **Baseline (the ratchet):** the ACL is the first service with branching logic, so it
 sets the repo-wide baseline. Observed score **95%**; enforced `break` threshold **90%**

services/acl/Acl.slnx

@@ -1,7 +1,9 @@
 <Solution>
+  <!-- Stryker-only. Acl.IntegrationTests is left out on purpose: it needs a live
+       OpenZaak, so in the mutation job it fails its initial run (#174). The root
+       register-referentie.slnx still builds and lints it. -->
   <Project Path="Acl.Api/Acl.Api.csproj" />
   <Project Path="Acl.Application/Acl.Application.csproj" />
   <Project Path="Acl.Infrastructure/Acl.Infrastructure.csproj" />
-  <Project Path="Acl.IntegrationTests/Acl.IntegrationTests.csproj" />
   <Project Path="Acl.Tests/Acl.Tests.csproj" />
 </Solution>