Expand negative test vectors v0.9

[DigiEmu]

Summary

This PR expands docs/NEGATIVE_TEST_VECTORS_v0.9.md.

The previous file accidentally contained the PR summary instead of the full negative test vector document.

This PR replaces it with the full public review draft, including:

• purpose and negative outcome definitions
• base positive test vector
• FAIL vectors for changed decision, risk level, and policy reference
• ERROR vectors for malformed JSON, missing snapshot, missing expected hash, unsupported hash algorithm, unsupported canonicalization, and invalid field type
• expected reason codes
• conformance relevance
• relationship to Test Vectors v0.9
• relationship to Verify Report Schema v0.9

Why this matters

Negative test vectors are needed to verify that implementations correctly distinguish:

PASS  = computed hash matches expected hash
FAIL  = processable input with mismatched hash
ERROR = verification could not be completed