docs: explain synthetic verification fixtures

[salismidtrans]

What changed

• Adds a concise root README section explaining how the tracked synthetic fixtures verify Midtrans CLI behavior locally.
• Adds testdata/README.md with the fixture inventory, test-only BI-SNAP key warning, contributor safety rules, and proof-level boundary.
• Adds a release contract test that prevents fixture-backed tests from being presented as merchant, provider, or real Sandbox proof.

Why

The public repository already tracks deterministic payloads, generated cryptographic test vectors, and complete/broken merchant-repository shapes. Merchants evaluating the public preview need to know what these fixtures validate and, equally importantly, what they cannot prove.

Safety boundary

The fixtures contain synthetic identifiers, deterministic signatures, empty credential placeholders, and a deliberately public test-only RSA key pair. They must never be reused in a merchant environment. Passing fixture-backed tests is local deterministic CLI proof only; real Sandbox and provider proof still require merchant-owned credentials, activation, callbacks, and provider interaction.

Validation

• TDD red-green fixture-documentation contract
• go test ./... -count=1
• Focused fixture-consuming package and end-to-end tests
• Full ./tools/check_release.sh
• Gitleaks history scan across 110 commits
• Public-release boundary and relative-link checks
• Commit and pre-push security hooks

All checks passed on commit 20f79217b238168750e8322af0621c269c3d0f23.