[V01] Trust fixtures

[yunseo-kim]

Scope

Complete common provenance, policy, identity, trust-root, transparency, signing-time, and diagnostics fixtures.

Contracts

ADRs 0028, 0037, 0061, 0062, 0068-0071

Dependencies

L04

Blocking issues: [L04] #42.

Executable QA

`go run ./cmd/slsa-builder-internal fixture-check --index testdata/fixtures/index.json --suite trust` must exit 0 with accepted count equal to the index and rejected count equal to the index. `go test ./internal/attestation ./internal/identity ./internal/policy -run '^TestFixtureCorpus$' -v` must exit 0.