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/advisories/explorer-integration.md

@@ -16,4 +16,9 @@
 - Provenance: DSSE/Rekor optional; bundle manifests.
 - Cross-links to findings ledger and VEX decisions.
 
+### Hash Capture Checklist (when spec arrives)
+- `assets/vuln-explorer/advisory-normalized.json`
+- `assets/vuln-explorer/advisory-withdrawn.json`
+- `assets/vuln-explorer/advisory-bundle-manifest.json`
+
 _Last updated: 2025-12-05 (UTC)_