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.

.gitea/workflows/signals-evidence-locker.yml

@@ -17,6 +17,9 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v4
 
+      - name: Task Pack offline bundle fixtures
+        run: python3 scripts/packs/run-fixtures-check.sh
+
       - name: Build deterministic signals evidence tar
         run: |
           set -euo pipefail