test(acl): add Stryker config + mutation make target recording the 95% baseline (refs #47)

Configure Stryker.NET for the ACL in solution mode (Acl.slnx), so both
Acl.Application and Acl.Infrastructure — the two projects under test — are
mutated while Acl.Api (untested) is skipped. Record the repo-wide mutation
baseline as the ratchet (CLAUDE.md §5): observed score 95%, enforced break
threshold 90% (one-mutant headroom over the ~20-mutant surface). The ACL is the
first service with branching logic, so it sets the baseline; later slices
ratchet it up deliberately, never down.

Add a `mutation` make target (`dotnet tool restore` + `dotnet stryker`) and wire
it into the `make ci` aggregate, keeping `make ci` an exact mirror of the
pipeline.

Refs #47.

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

services/acl/stryker-config.json

@@ -0,0 +1,11 @@
+{
+  "stryker-config": {
+    "solution": "Acl.slnx",
+    "reporters": ["progress", "html"],
+    "thresholds": {
+      "high": 95,
+      "low": 90,
+      "break": 90
+    }
+  }
+}