Add MongoDB storage library and update acceptance tests with deterministic stubs

- Created StellaOps.Notify.Storage.Mongo project with initial configuration.
- Added expected output files for acceptance tests (at1.txt to at10.txt).
- Added fixture input files for acceptance tests (at1 to at10).
- Created input and signature files for test cases fc1 to fc5.

docs/observability/vuln-telemetry.md

@@ -15,4 +15,9 @@
 - Traces: key spans; sampling guidance.
 - Dashboards: to be added with hashes.
 
+### Hash Capture Checklist (when telemetry plan arrives)
+- `assets/vuln-explorer/metrics-sample.json` (scrape example)
+- `assets/vuln-explorer/logs-sample.jsonl` (structured log snippet)
+- `assets/vuln-explorer/traces-sample.json` (span export)
+- `assets/vuln-explorer/dashboard.json` (dashboard export)
 _Last updated: 2025-12-05 (UTC)_