Validate saved baselines before comparison

[rogerchappel]

Summary

• validate version-1 baseline structure and fields before comparison
• return stable stack-free diagnostics in text and JSON CLI modes
• document the accepted baseline contract

Commits

• 326f5ee test: cover invalid baseline diagnostics
• 13751a7 fix: validate loaded baseline files
• e422172 docs: define baseline version validation

Verification

• npm ci
• npm run release:check (99 tests; audit, typecheck, lint, coverage, smoke, package smoke)
• git diff --check origin/main...HEAD

All commits use Roger Chappel <miscanalysis@gmail.com> as author and committer.

[rogerchappel]

Automated merge note\n\nTriage class: auto-merge\n\nSummary: Validates persisted baseline JSON with deterministic user-facing diagnostics, documentation, and regression coverage.\n\nChecks run: GitHub CI test matrix SUCCESS (Node 20, 22, 24); isolated-worktree npm ci, npm run release:check (99 tests plus audit, typecheck, lint, smoke, and package smoke), and git diff --check passed.\n\nRebased/CI-repaired: no.\n\nVerified head SHA: e422172.