test: establish frontend gate taxonomy

docs/testing/taxonomy.md

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+# Test and evidence taxonomy
+
+Each gate is blocking in its declared scope. Failures are not downgraded with
+`continue-on-error` or warning-only scripts.
+
+| Level | Command | Evidence |
+| --- | --- | --- |
+| runtime schema | `pnpm test:runtime-schema` | `artifacts/tests/runtime-schema.xml` |
+| unit | `pnpm test:unit` | `artifacts/tests/unit.xml` |
+| component | `pnpm test:component` | `artifacts/tests/component.xml` |
+| integration | `pnpm test:integration` | `artifacts/tests/integration.xml` |
+| end-to-end | `pnpm test:e2e` | `artifacts/tests/e2e/` |
+| accessibility | `pnpm test:a11y` | `artifacts/tests/a11y.json` |
+
+A control is verified only when a positive fixture passes and its deliberately
+failing negative fixture is rejected. Generated evidence is retained by CI;
+the repository tracks only the evidence directory structure.
+
+Promotion is an AND graph:
+
+1. merge gates
+2. merge gates plus release gates
+3. release gates plus rollback/runbook drills
+4. production promotion plus eligible field Web Vitals evidence