feat: type-safe router labels via per-label userData map

[B4nan]

What

Lets users declare a label → userData map once and pass it as the router's Routes type argument, so route handlers get request.userData typed by label — and unknown labels become a compile-time error.

interface Routes {
    PRODUCT: { sku: string; price: number };
    CATEGORY: { categoryId: string };
}

const router = createCheerioRouter<CheerioCrawlingContext, Routes>();

router.addHandler('PRODUCT', async ({ request }) => {
    request.userData.sku;   // string
    request.userData.price; // number
});

router.addHandler('TYPO', async () => {}); // ❌ compile error: not a known label

How

• New Routes generic on Router, RouterHandler and RouterRoutes, defaulting to an open Record<string, …> so existing untyped usage is unchanged.
• Router.create and all 10 createXRouter factories expose two overloads — a route-map form (per-label typing) and the legacy flat-userData form. The second type argument selects between them: a route map matches the first overload, any other shape falls through to the second. This makes the documented createXRouter<Ctx, Routes>() form work without changing the meaning of the released <Ctx, UserData> type argument, keeping it fully backwards compatible. (The sole ambiguous case — a flat userData whose every field is itself an object — is read as a route map; a single scalar field disambiguates it.)
• addHandler gains two overloads: a label-strict one (infers userData from the label) tried first, and the existing <UserData>-generic one as the fallback. The default handler receives the union of all declared userData shapes.
• The route-map constraint uses the F-bounded Record<keyof Routes, Dictionary> form so both interface and type route maps are accepted (a plain Record<string, …> constraint silently rejects interface declarations).

This is a compile-time-only change (zero runtime cost). A follow-up targeting v4 adds an opt-in Standard Schema variant that also validates userData at runtime.

Relates to #3082

🤖 Generated with Claude Code

[janbuchar]

LGTM, please address the one docs-related comment.