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/vex/explorer-integration.md

@@ -20,4 +20,9 @@
 ## Determinism
 - Use fixed CSAF samples; hash examples.
 
+### Hash Capture Checklist (when assets land)
+- `assets/vuln-explorer/vex-csaf-sample.json` (input)
+- `assets/vuln-explorer/vex-mapping-output.json` (normalized decisions)
+- `assets/vuln-explorer/vex-precedence-table.md` (suppression/precedence matrix)
+
 _Last updated: 2025-12-05 (UTC)_