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+## Summary
+
+This PR adds `docs/NEGATIVE_TEST_VECTORS_v0.9.md`.
+
+The new public review draft defines intentional FAIL and ERROR test vectors for DigiEmu Core verification.
+
+It includes:
+
+- tampered snapshot FAIL cases
+- malformed JSON ERROR case
+- missing snapshot ERROR case
+- missing expected hash ERROR case
+- unsupported hash algorithm ERROR case
+- unsupported canonicalization ERROR case
+- invalid field type case
+- expected reason codes
+- conformance relevance
+
+## Why this matters
+
+DigiEmu Core should not only verify valid snapshots.
+
+Implementations must also correctly distinguish:
+
+```text
+PASS  = computed hash matches expected hash
+FAIL  = processable input with mismatched hash
+ERROR = verification could not be completed
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