Read implicitly-returns-nil per RBS overload

[apiology]

Array#max(1) comes back with a spurious , nil appended, so this fails at --level strong:

# @return [Array<String>]
def probe
  a = ['x', 'y']
  a.max(1)
end

# probe.rb:2: Declared return type ::Array<::String> does not match
#             inferred type ::Array, nil for #probe

RBS declares no nil there. %a{implicitly-returns-nil} is attached to an individual overload, and rbs 4.1.3's core/array.rbs puts it only on the zero-argument ones:

def max: %a{implicitly-returns-nil} () -> E
       | %a{implicitly-returns-nil} () { (E a, E b) -> _CompareToZero } -> E
       | %a{implicitly-returns-nil} %a{warning: ...} () { (E a, E b) -> _CompareToZero? } -> E
       | (::int count) -> ::Array[E]
       | (::int count) { (E a, E b) -> _CompareToZero } -> ::Array[E]
       | %a{warning: ...} (::int count) { (E a, E b) -> _CompareToZero? } -> ::Array[E]

The annotation is also no longer passed down to block return types: it describes what the method returns, not what the block returns, and a block that may return nil says so with ? ��� as the third and sixth overloads above do.

This PR was written by Claude Code on behalf of @apiology.