release: publish-gated append-only provenance ledger for src tarballs

[ronaldtse]

The dangling case (issue #100), reproduced on main

tebako-runtime-ruby v0.16.11's manifest records its 3.3.12 build input as:

{ "release": "spec22-chain",
  "sources": [ { "name": "tfs-ruby-3.3.12-src.tar.gz",
                 "sha256": "a54f25657ae219fb5e0cdf1aca5b695320a2963fa8e242b7c106d3d01d2d8f5e" } ] }

• The recorded release name dangles outright:
  gh api repos/tamatebako/ruby/releases/tags/spec22-chain → 404 (deleted chain mirror).
• The recorded digest matches none of the issue's v0.2.21–v0.2.27 window. Scanning every
  surviving release's SHA256SUMS by hand (39 releases) shows v0.2.28 did carry those bytes — the
  issue's table predates v0.2.28 by hours. So the digest is verifiable today only by manual
  full-history scan, and only while v0.2.28 keeps surviving: nothing stops a re-roll or a
  release cleanup from silently orphaning it tomorrow.

The model: provenance by content digest + a cumulative, append-only ledger

The durable reference is the tarball's sha256 — content-addressed by the deterministic-roll
rule and already recorded downstream as built_from.sources[].sha256. A release name stays
what it is: fetch metadata. What was missing is a permanent attestation that the factory
published a digest.

Every release now carries provenance.yaml: the cumulative, append-only ledger of every
(asset, sha256) the factory has ever published, each with the release that first carried those
bytes. release-src's publish job builds it (tools/provenance --build), chained from the newest
existing release's ledger — bootstrapped from every published SHA256SUMS while none carries one
(the one-time recovery path) — and gates the release on:

• append-only — the previous ledger is an exact prefix of the new one: a re-rolled tarball
  appends an entry; a published digest is never dropped, reordered, or re-attributed;
• completeness — every asset of the release being published is recorded.

Either failure fails the publish with a named error. The ledger chains through the newest
existing release, so factory releases must never be deleted — deleting the newest one breaks
the chain and blocks the next publish loudly (documented in README § "Provenance ledger").

After: the same lookups against the ledger (run live from this branch)

$ tools/provenance --build --tag v0.2.39 --sums <v0.2.38 SHA256SUMS> --out provenance.yaml
provenance: bootstrap over 39 published releases (4553 entries; no previous ledger to prefix-check)
provenance: v0.2.39 adds 0 entries (of 149 assets); ledger now 4553 entries
provenance: append-only + completeness assertions passed; wrote provenance.yaml

$ tools/provenance --verify a54f25657ae219fb5e0cdf1aca5b695320a2963fa8e242b7c106d3d01d2d8f5e --ledger provenance.yaml
a54f25657ae219fb5e0cdf1aca5b695320a2963fa8e242b7c106d3d01d2d8f5e  tfs-ruby-3.3.12-src.tar.gz  (first published: v0.2.28)

$ tools/provenance --verify 38d41d860947cce7e13b509e16acafac8af70a09b7081218dc31a38808372f6f --ledger provenance.yaml
38d41d860947cce7e13b509e16acafac8af70a09b7081218dc31a38808372f6f  tfs-ruby-3.3.12-src.tar.gz  (first published: v0.2.21)
# ^ v0.2.21's bytes, re-rolled away — no current release carries them; the ledger still attests.

$ tools/provenance --verify deadbeef…deadbeef --ledger provenance.yaml
tools/provenance: sha256 deadbeef… is unattested in provenance.yaml: no factory release ever
published these bytes — provenance is unresolvable, do not trust a build input claiming it
(exit 1)

A runtime's recorded built_from.sources[].sha256 now resolves against the provenance.yaml of
its own built_from.release — or of the newest release; the ledger only ever grows.

What changed

• tools/lib/tfs/provenance.rb — Tfs::Provenance, the pure ledger model: strict parse/serialize,
  SHA256SUMS → entries, chaining (first appearance owns first_release), the append-only and
  completeness assertions, and the audit lookup. Every violation is a named error.
• tools/lib/tfs/provenance_feed.rb — Tfs::ProvenanceFeed, the releases-API/download glue
  (injectable fetcher; drafts excluded, oldest-first).
• tools/lib/tfs/http_get.rb — optional headers: kwarg (the releases API requires a
  User-Agent; GH_TOKEN/GITHUB_TOKEN authorizes when present). Backward compatible.
• tools/provenance — --build (publish gate) and --verify <sha256> [--ledger file|-] (audit).
• .github/workflows/release-src.yml — the publish job checks out the repo, builds + checks the
  ledger, and attaches dist/provenance.yaml to the release.
• README.md — new "Provenance ledger" section: the model, the two gate assertions, the audit
  command, and the never-delete-releases rule.

Consumer impact (tebako-runtime-ruby)

None required — no field moves. built_from.sources[].sha256 was already the digest; this change
makes it durably verifiable. The fetch path (named assets + SHA256SUMS against the pinned release)
is untouched; provenance.yaml is an additive asset.

Verification

• bundle exec rspec — 149 examples, 0 failures (121 → +28: ledger model + feed specs).
• bundle exec tools/validate_manifests, bundle exec tools/validate_msys_pairs — green.
• Live bootstrap against the production releases API (the output above).
• First real release after merge bootstraps and attaches the ledger; every later release chains.

Closes #100