test(domain): mutation baseline 90 (achieved 97.7%) + CI/Makefile wiring (refs #6)

Stryker.NET config for the domain service (break 90, the repo's ratchet floor),
excluding the OpenZaakJobPump hosted-shell from mutation. Hardened the unit tests
to kill survivors — Basic-credential value, variable types, null/failure response
paths, option defaults, guard clauses, save counts and log output — leaving only
two documented equivalent mutants (Stryker-disabled). make mutation runs the domain
ratchet and CI uploads its report alongside the others.

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

services/domain/Big.Infrastructure/FlowableWorkflowClient.cs

@@ -102,6 +102,8 @@ public sealed class FlowableWorkflowClient(HttpClient http, FlowableOptions opti
     {
         /// <summary>The registration id this job carries as a process variable.</summary>
         public string RegistrationId() =>
+            // Stryker disable once Linq : equivalent — Single and SingleOrDefault both throw on a job
+            // with no registrationId variable (the only untested branch), so the mutant is indistinguishable.
             Variables.SingleOrDefault(v => v.Name == "registrationId")?.Value
             ?? throw new InvalidOperationException($"OpenZaakAanmaken job {Id} carries no registrationId variable.");
     }