Improve option and result type precision - #613

With the covariant option and result types in place, several annotations can now say precisely
what the code does. The five concrete adapters declare create() as returning an option of the
adapter itself rather than of the interface, which removes the widening casts inside them and
gives callers the concrete type. The repository builder's reader, writer and adapter parameters
now document the actual acceptance, an instance of the respective interface or a class-string of
an adapter, which matches the runtime guards exactly and lets the builder closures type-check
without laundering. The string and regex helpers declare the non-negative integer ranges they
actually produce, correcting a wrong-shaped cast in Str::pos. None of this changes behaviour, and
code typed against the old signatures only ever sees narrower types. The phpstan baseline shrinks
from sixteen entries to five as a byproduct.