Add negative test vectors v0.9

[DigiEmu]

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:

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