test(event-subscriber): ratchet projector mutation baseline to 100% (refs #7)

Sharpen the projector tests so Stryker has no survivors (was 75%): assert a replayed
delivery never reaches the store (upsert count, not just row count), that two distinct
zaken get distinct rows (pins the idempotency key), that rebuild clears stale rows, and
a Theory over wrong kanaal/resource/actie combinations (pins the zaken/zaak/create guard).
Add the per-service Stryker config + solution; break threshold 90 (CLAUDE.md §5).

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

services/event-subscriber/EventSubscriber.Tests/InMemoryStores.cs

@@ -20,8 +20,13 @@ internal sealed class InMemoryProjectionStore : IProjectionStore
 {
     private readonly Dictionary<string, RegisterEntry> _byId = [];
 
+    /// <summary>How many times a write was attempted — lets a test prove a deduped delivery
+    /// never reaches the store (an idempotent upsert hides the difference in row count alone).</summary>
+    public int UpsertCount { get; private set; }
+
     public Task UpsertAsync(RegisterEntry entry, CancellationToken ct = default)
     {
+        UpsertCount++;
         _byId[entry.Id] = entry;
         return Task.CompletedTask;
     }