Change $computed return type from ComputedMarker<R> to R to allow
direct property assignment without type casts.

The runtime behavior remains unchanged - a marker object is returned
and unwrapped by the store's set handler. The type change enables:

  store.$.doubled = store.$computed(($) => $.count * 2);

without requiring 'as any' or 'as number' casts.

Updated documentation explains this design decision.