Conformance: wire go-stack to consume conformance vectors from ts-stack

[sirdeggen]

ts-stack publishes conformance vectors as a GitHub Actions artifact (90-day retention) on every push to main. go-stack should have a conformance CI job that: downloads the vectors artifact from ts-stack, runs each domain's tests against them. The plan is documented in ts-stack/conformance/GO_PLAN.md. Two consumption options: (1) dawidd6/action-download-artifact to pull the artifact, or (2) sparse checkout of ts-stack conformance/vectors/. Start with the SDK domain (go-sdk) as the pilot — it has the most coverage in the vector set.