feat: add release conformance attestation format (relay-conform-attestation/1), spec §20.6

[SoundMatt]

Summary

Implements a scoped-down version of issue #142 (NEW-SPEC-4), per explicit direction to scope this spec-only rather than stand up real signing/publishing infrastructure.

Scope decision

The originating issue proposed a signed SLSA/in-toto attestation published alongside a release's container image, with its digest surfaced in version --format json and relay probe/relay report treating unverifiable releases as non-conformant. This PR ships spec-only: it defines the predicate format an implementation MAY publish, without actual signing or publishing.

Two concrete blockers made "full implementation" the wrong call here:

• RELAY's own CI only does docker build as a smoke test — it doesn't push images to any registry. "Published alongside the release's container image" presupposes infrastructure that doesn't exist.
• Real cryptographic signing needs real key management (e.g. Sigstore/cosign keyless signing). Fabricating an unreal "signature" would be worse than not signing at all.

predicate.signed MUST be false on an unsigned attestation. This section defines shape only — it does not mandate a signing mechanism or publication channel.

What's added

• New §20.6 defines the relay-conform-attestation/1 predicate: an in-toto Statement whose subject digest is the SHA-256 of the attested binary file, and whose predicate bundles the full §17.2 conformance manifest, the attestation-generating tool's own embedded vectors_version (§15.8 — explicitly not observed from the attested binary, since a black-box invocation cannot introspect it), and an optional implementation-defined safety_evidence_summary.
• New embedded schema spec/schemas/relay-conform-attestation.json.
• Reference implementation: relay conform --attestation <binary> in cmd/relay/conform.go.
• New REQ-RELAY-098.

Bug found and fixed along the way

While wiring the attestation's conformance_manifest field, I found buildManifest's Requirements slice was missing entry 16 (Vector manifest) — PR #181 added the §17 Requirement 16 spec text and verifier-table row but never added the corresponding code entry. Every relay-conform/1 manifest generated since v2.4 has silently omitted it, despite the spec's own text mandating "exactly one entry per §17 requirement (1–16, ...)". Fixed; the two manifest-shape tests that should have caught this (TestBuildManifestSelf, TestRunConformManifestFlag) were also stale at a hardcoded 15 and are corrected alongside it.

Version

SpecVersion bumped 2.5 → 2.6 (MINOR, new optional format + CLI flag).

Verification

• 6 new tests for the attestation feature, including independent cross-checks: the subject digest matches a separately-computed SHA-256 of the binary file, vectors_version matches the tool's own parsed vectors_manifest.json, and schema validation against the new embedded schema.
• Real mutation test on the Requirement-16 bug fix: reverted the fix, confirmed both TestBuildManifestSelf and TestRunConformManifestFlag correctly failed, restored and confirmed green.
• Manually verified relay conform --attestation end-to-end: digest independently matched via hashlib.sha256, overall: PASS, signed: false, 16 requirement entries.
• go build ./... && go vet ./... && go test ./... — all packages green.
• gofmt -l . and golangci-lint run (latest) — clean.
• gofusa check (pinned v0.48.0): 0 errors. gofusa trace -req-coverage 100: 100% (98/98).

Closes #142.