Resolve the strictness baseline entries at source

[GrahamCampbell]

Four of the five phpstan baseline entries point at real, if harmless, looseness in the code
rather than at annotation gaps, so this resolves them at source. The lexer now compares the
preg_match result against one explicitly, which throws in exactly the same cases as the previous
falsy check. The byte order mark comparison in Str::utf8 uses strict equality, which agrees with
the loose comparison for every value substr can produce there. The environment adapters split
their read chains so the scalar filter's guarantee is stated once as a checked narrowing instead
of an unverifiable string cast on the mapped result. The regenerated baseline shrinks to the
single remaining lexer entry, where phpstan cannot infer the type of the static pattern cache.