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.Tests/CapturingLogger.cs

@@ -0,0 +1,24 @@
+using Microsoft.Extensions.Logging;
+
+namespace Big.Tests;
+
+/// <summary>A minimal <see cref="ILogger{T}"/> that records the level and rendered message of each
+/// entry, so tests can assert that (for example) a failing job was logged.</summary>
+internal sealed class CapturingLogger<T> : ILogger<T>
+{
+    public List<(LogLevel Level, string Message)> Entries { get; } = [];
+
+    public IDisposable BeginScope<TState>(TState state) where TState : notnull => NullScope.Instance;
+
+    public bool IsEnabled(LogLevel logLevel) => true;
+
+    public void Log<TState>(LogLevel logLevel, EventId eventId, TState state, Exception? exception,
+        Func<TState, Exception?, string> formatter)
+        => Entries.Add((logLevel, formatter(state, exception)));
+
+    private sealed class NullScope : IDisposable
+    {
+        public static readonly NullScope Instance = new();
+        public void Dispose() { }
+    }
+}