Type Kernel#<=> as returning Integer?

[apiology]

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

Problem: Steep rejects an ordinary <=> whose class's RBS does not redeclare it.

class Version
  attr_reader :major, :minor
  def <=>(other) = [major, minor] <=> [other.major, other.minor]
end

lib/version.rb:9:6: [error] Cannot allow method body have type `(::Integer | nil)` because declared as type `(0 | nil)`
│ Diagnostic ID: Ruby::MethodBodyTypeMismatch

Any class implementing <=> without redeclaring it inherits Kernel#<=>'s 0?, which only Object's default satisfies, so delegating to Array#<=> or Integer#<=> (typed Integer?) is an error.

Solution: Type Kernel#<=> as Integer?, the type those comparisons have and what it was before #1945, which still covers the default's 0 or nil.