Support navigating to external URLs

## Context (updated for current API)
`@effector/router` has **no `redirect()` helper** — the original atomic-router framing doesn't map. Navigation goes through `controls.navigate` / `router.navigate` (`NavigatePayload = { path?, query, replace? }`, see `packages/core/lib/types.ts`), which only builds internal paths.

## Request
Provide an explicit, supported way to navigate to an absolute external URL (e.g. `https://example.com/...`) without breaking internal-route navigation.

## Possible directions
- An explicit external-navigation API (separate from internal `navigate`), or an `external` flag on the navigation payload.
- Keep internal path building untouched; external navigation must be opt-in and obvious at the call site.

## Traceability
Source: atomic-router/atomic-router#46 — https://github.com/atomic-router/atomic-router/issues/46