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/scripts/sbom-vex/README.md

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+# SBOM→VEX Offline Kit (Stub)
+
+This kit supports sprint task 6 (SBOM-VEX-GAPS-300-013).
+
+Contents (stub):
+- `verify.sh` – chain hash stub for SBOM + DSSE + Rekor + VEX
+- `chain-hash-recipe.md` – canonicalisation steps
+- `inputs.lock` – pinned tool versions and snapshot
+- `proof-manifest.json` – chain hash placeholder
+- `sbom-vex-blueprint.svg` – diagram placeholder
+
+Next steps:
+- Add real SBOM/VEX samples and Rekor bundle snapshot.
+- Produce DSSE signatures for proof manifest and scripts.
+- Include time-anchor and backpressure/error policy notes per BP1–BP10.