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/stryker-config.json

@@ -0,0 +1,12 @@
+{
+  "stryker-config": {
+    "solution": "EventSubscriber.slnx",
+    "test-projects": ["EventSubscriber.Tests/EventSubscriber.Tests.csproj"],
+    "reporters": ["progress", "html"],
+    "thresholds": {
+      "high": 95,
+      "low": 90,
+      "break": 90
+    }
+  }
+}