docs: scaffold vuln parity assets folder and sample placeholders

docs/db/reports/assets/vuln-parity-20251211/README.md

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+This folder holds frozen inputs for the 2025-12-11 Vulnerability parity run (Mongo vs Postgres).
+
+Drop files here and record their SHA256 in the parity report tables:
+- sboms/: SBOM samples
+- advisories/: advisory export subset (10k) if used
+- hashes.sha256: manifest of all files
+
+Do not modify contents once hashes are recorded.

docs/db/reports/assets/vuln-parity-20251211/hashes.sha256

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+# populate after files are added