SmrSector: narrow doc type for oppositeDir argument
SmrSector: narrow doc type for oppositeDir argument

The `$dir` argument can only be one of four values (i.e. we remove the
"Warp" match arm), because it is only used in functions that support
setting the neighbors in the `sector` database, which is distinct from
warps.

This fixes PHPStan warning:

> Match expression does not handle remaining value: string

hemberger/smr,hemberger/smr,smrealms/smr,smrealms/smr,hemberger/smr,hemberger/smr,smrealms/smr