ci: SLSA L3 build-provenance attestations on the publish arm

[ronaldtse]

What

Fans out the SLSA Build L3 artifact-attestation arm that landed in tamatebako/tebako#694 to this factory's publish path (unblocked now that #226 has merged).

• .github/workflows/_build-platform.yml — the per-leg publish job (the shipping leg: Publish the leg's runtime package (spec 13 §2a) + Sign the leg's release assets (spec 09 §5)) gains a terminal Attest build provenance step (actions/attest-build-provenance@4d101475d8b20a2381f78447822ac1eab6504dd8, v4.2.2 — the same pin tebako's release.yml carries) with subject-path: runtime-packages/*: exactly the staged bytes the uploader ships — the runtime package (exe + env image), its .sha256 sidecars, the .manifest.json shard, plus the detached .asc set when the signing arm ran (the gem's signer converges them into runtime-packages/, its default local dir). The dot-hidden .leg-complete marker is never globbed, and the non-shipped *-ucrt-ruby*.dll boot-smoke copies are rm'd by the publish step before the attest runs.
• The publish job declares attestations: write + id-token: write; .github/workflows/publish.yml's four platform caller jobs (windows, linux-gnu, linux-musl, macos) grant attestations: write down to the reusable workflow — the same flow as the existing windows-signing id-token: write grant (a caller's permissions cap the callee's).
• The coordinator's release job (registry render + bot PRs) is untouched: the registry lands on main by PR, never as a release asset, so there is nothing to attest there.

Gate parity

The attest step carries the same gate as the leg's publish/sign steps: the publish job's own

if: ${{ always() && !cancelled() && needs.compute.outputs.run == 'true' && inputs.publish && !inputs.audit }}

Build CI, audit runs, and non-publish dispatches skip the publish job outright, so they never attest. A leg whose build failed dies at the artifact-completeness guard before the attest step.

Verification

• python3 -c yaml.safe_load on both touched workflows: parse OK.
• actionlint .github/workflows/publish.yml .github/workflows/_build-platform.yml: identical findings to main (four pre-existing shellcheck notes in unrelated steps; this change adds none).
• bundle exec rspec: 295 examples, 0 failures, 38 pending (environmental — unchanged from main). spec/build_workflow_spec.rb (the ci: run the spawn-edge acceptance per leg before publish (#211) #226 step-ordering assertions): 22 examples, 0 failures — the publish-job ordering contract (download → guard → publish → sign) is preserved; the attest step appends after sign, so no spec change was needed.

Consumers verify with gh attestation verify <asset> --repo tamatebako/tebako-runtime-ruby.

Not to be merged by the author — the orchestrator merges after checks go green.