tests: move screenshot baselines from git to R2 (content-addressed manifest v2)

Baseline PNGs (492, 35 MB) leave git: they now live in the private R2 bucket
pcbjam-ci-screenshots as immutable sha256/<hex>.png objects, pinned by the
committed screenshot-manifest.json (v2: name, engine, sha256, bytes, dims).
tests/baseline-screenshots/ becomes a gitignored cache materialized by the new
`npm run screenshots:fetch` (r2-sync.ts + aws4fetch r2-store.ts).

- promote.ts: requires the RW keypair (shell env or gitignored tests/.env),
  syncs the cache, uploads new hashes BEFORE rewriting the manifest; the
  manifest diff is the only git-visible output. --prune only edits the
  manifest — R2 objects are never deleted, old commits still resolve.
- compare.ts: skips the gate (exit 0, no report.json) when the manifest
  expects baselines but the cache is empty (secretless callers).
- changelog.ts: diffs the manifest between revs and fetches bytes from R2;
  guards against the migration commit (base manifest not v2 → skip).
- gen-manifest --check: v2 schema + resurrection guard (fails if baseline
  PNGs are ever re-committed); credential-free so every caller can gate.
- wasm-build.yml: declares optional read-only S3 secrets, caches + fetches
  baselines before the lint gate; release.yml/deploy-staging.yml pass them.
- screenshot-changelog.yml: triggers on the manifest path instead of PNGs.

All 492 objects are seeded and hash-verified in the bucket; fetch/compare
degrade to a warn-and-skip without credentials.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/.gitignore

@@ -1,6 +1,8 @@
 # Screenshot model: specs capture via stableShot() → page.screenshot into test-results/, which is
-# transient and gitignored at the repo root (/tests/test-results/). The committed baselines live in
-# tests/baseline-screenshots/ (+ 3d-regression/, gal-regression/), authored from CI's deterministic
-# Linux render and diffed OFFLINE by tools/screenshots. There are no Playwright-native
-# *-snapshots/ baselines — Playwright does no inline screenshot comparison.
+# transient and gitignored at the repo root (/tests/test-results/). Baselines live in a private R2
+# bucket, content-addressed by sha256 and pinned by the committed screenshot-manifest.json;
+# tests/baseline-screenshots/ is a gitignored local cache (`npm run screenshots:fetch`), authored
+# from CI's deterministic Linux render and diffed OFFLINE by tools/screenshots. (3d-regression/ and
+# gal-regression/ baselines are still committed.) There are no Playwright-native *-snapshots/
+# baselines — Playwright does no inline screenshot comparison.
 logs/