docs(arts): B3 — point the runtime-artifact shortcut at the composition root

The "I want to… use a runtime artifact" row in docs/README.md now routes
through the composition root (architecture/active-app.md) → the arts map
(arts/README.md) → the per-artifact README. Map/aliases were already complete
(9e48b80b). docs:check 0 errors.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/README.md

@@ -128,7 +128,7 @@ and the server-authoritative engines in `src/svrs/`.
 | Theme it / add a token | [`theming/reference.md`](./theming/reference.md) + [`canon/tsc.md`](./canon/tsc.md) |
 | Animate it (motion · loops · stagger · reduced-motion) | [`theming/motion-guide.md`](./theming/motion-guide.md) |
 | Understand why a decision was made | `docs/decisions.md` → the relevant RFC / [`decisions/book-deviations.md`](./decisions/book-deviations.md) |
-| Use a runtime artifact (auth, cache, http, …) | `arts/README.md` + `src/arts/{name}/README.md` |
+| Use a runtime artifact (auth, cache, http, …) | [`architecture/active-app.md`](./architecture/active-app.md) (the composition root) → [`arts/README.md`](../src/arts/README.md) (the map) → `src/arts/{name}/README.md` (per-artifact) |
 | **Write or edit documentation** | [`docs/authoring.md`](./authoring.md) — the authoring rules |
 | Test or validate a change | [`docs/testing-and-tooling.md`](./testing-and-tooling.md) — tests, validators, codegen, SSR |