Implement cross-browser screenshot baseline storage (RFC 0002)

[PAMulligan]

Summary

Implement the cross-browser screenshot baseline-storage design decided in
RFC 0002 — Cross-Browser Screenshot Baseline Storage.

This is the implementation follow-up for the decision issue #85 (AC #2:
"Implementation tracked as a follow-up issue"). Blocked on RFC 0002 acceptance —
no code lands until the RFC Status moves Proposed → Accepted.

Decision recap

• Default backend: commit baselines to the repo (ratify the current
  .gitignore behavior), extended to firefox/webkit; storage: "lfs" flag for
  large baseline sets.
• Pluggable visualBaselines.backend: commit (default) | ci-artifact |
  service (Chromatic/Percy), behind a uniform adapter contract.
• Determinism precondition: capture cross-browser baselines in a pinned
  Playwright container; record + verify per-baseline provenance.

Deliverables (phased per RFC §10)

Phase A — make the signal real (non-blocking)

• Add the visualBaselines section to .claude/pipeline.config.json and its
  JSON Schema (strict additionalProperties: false).
• Add the provenance manifest (baselines/manifest.json) — engine, Playwright
  version, container image, sha256, gitSha, host.
• Capture and diff firefox/webkit from committed baselines (extend
  scripts/regression-test.sh or add scripts/cross-browser-baseline.sh),
  reusing scripts/visual-diff.js at threshold 0.03.
• Fix the mislabeled orchestration.phases.cross-browser so the phase performs
  the comparison it advertises (replaces capture-only
  scripts/cross-browser-test.sh behavior).
• Docs: update CLAUDE.md + a docs/ page for the cross-browser baseline flow.

Phase B — determinism, then teeth

• Pin the Playwright container for capture in CI and locally (docker run).
• Enforce provenance in the comparator (flag stale/foreign baselines instead of
  emitting false pixel diffs).
• Once the measured false-positive rate is acceptable, allow blocking: true
  and flip qualityGate.crossBrowserScreenshotsRequired.

Phase C — the other adapters

• storage: "lfs" flag + .gitattributes automation + migration docs.
• ci-artifact adapter (compare against last-green main).
• service adapter (Chromatic/Percy) — token via CI secret, opt-in.

Open questions to resolve during implementation

See RFC §13: unify regressionTesting vs. keep separate; blocking threshold;
image-pin strategy; local firefox/webkit capture policy (warn vs. fail); LFS
history migration.

References

• RFC: docs/rfcs/0002-cross-browser-screenshot-baseline-storage.md
• Decision issue: Decide on cross-browser screenshot baseline storage #85
• Prior plans: docs/plans/2026-03-24-cross-browser-e2e-testing.md,
  docs/plans/2026-03-25-screenshot-regression-testing.md

Milestone: v2.0.0