[V05] CI fixture gates

[yunseo-kim]

Scope

Add required conformance CI: race tests, fixture harness, workflow checker, actionlint, offline tests, fuzz seeds, and scheduled fuzzing.

Contracts

ADRs 0005-0012, 0037; verification checklist

Dependencies

V01-V04

Blocking issues: [V01] #43; [V02] #44; [V03] #45; [V04] #46.

Executable QA

`actionlint .github/workflows/conformance.yml` must exit 0. `go test -race ./...` must exit 0. `go run ./cmd/slsa-builder-internal fixture-check --index testdata/fixtures/index.json` and `workflow-check --all .github/workflows` must both exit 0. `gh pr checks "$PR_NUMBER" --repo "$BUILDER_REPO" --watch` must exit 0 with all conformance jobs passing.