Build a community-curated adversarial test corpus across all 9 patterns

[linus10x]

ADR-0007 and ARCHITECTURE.md both name the adversarial test corpus as 'the single most-needed contribution.' A community-built corpus of adversarial inputs that exercises every veto reason code across all 9 patterns hardens v0.2 into v0.3.

How to contribute (per CONTRIBUTING.md):

1. Pick one veto reason code (e.g., FHA-VOUCHER, PROV-HASH-MISMATCH, RESIDENCY-CROSS-JURISDICTION-UNTAGGED)
2. Construct an adversarial input that exercises an edge case
3. Add it to tests/adversarial_corpus/<reason_code>.py
4. PR with the new test passing

Acceptance for issue close:

• At least 3 adversarial cases per reason code (≈45 cases total)
• All cases live in tests/adversarial_corpus/
• CI runs the corpus as a separate test selection (pytest tests/adversarial_corpus/)

Priority: P1. Composite 19/27. Community contribution magnet.