Replace untyped return types in PP with concrete types

[apiology]

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

Problem:

Most return types in stdlib/pp/0/pp.rbs are untyped, so values from PP lose their types:

s = PP.pp(obj, +"")      # untyped; PP.pp returns `out`, a String here
Foo.new.inspect          # untyped when `alias inspect pretty_print_inspect`; it returns a String

Solution:

Following lib/pp.rb (v0.6.4):

Ruby Method	untyped →	Basis
PP::_PrettyPrint#pretty_print, #pretty_print_cycle	void	called for their effect on the printer
PP::ObjectMixin#pretty_print, #pretty_print_cycle	void	same
PP::PPMethods#pp, #pp_object, #pp_hash	void	same
PP::ObjectMixin#pretty_print_inspect	String	returns PP.singleline_pp(self, ''.dup)
PP.pp, PP.singleline_pp	T (the out argument), or IO without out	both return out, which defaults to $>

void accepts any implementation return value, so existing pretty_print definitions (e.g. the -> untyped ones in stdlib/openssl) stay compatible.

PP.pp and PP.singleline_pp are split into two overloads, as in Marshal.dump, because a single ?T out overload leaves T unbound when out is omitted:

def self.pp: (_PrettyPrint obj) -> IO
           | [T < _LeftShift] (_PrettyPrint obj, T out, ?Integer width) -> T

Test plan

• bundle exec rake validate
• test/stdlib/PP_test.rb (also with --enable-frozen-string-literal), with added cases for PP.pp without out and with width
• bundle exec steep check
• bundle exec rubocop test/stdlib/PP_test.rb

🤖 Generated with Claude Code

https://claude.ai/code/session_01EhUSLUimEYJyCZv8NdQGKb

[ksss]

Thanks!