Replace deterministic untyped return types with concrete types

[zonuexe]

Summary

Several methods were typed -> untyped even though the implementation makes the return value deterministic. This replaces those with concrete types.

Ruby Method	untyped →	Basis
StringIO#putc	(Numeric | String)	returns its argument
Zlib::GzipWriter#putc	(Numeric | String)	returns its argument
Zlib::GzipFile#sync=	boolish	setter returns its argument
MonitorMixin::ConditionVariable#wait_for_cond	bool	C implementation returns Qtrue / Qfalse
MonitorMixin::ConditionVariable#wait	bool	delegates to #wait_for_cond
MonitorMixin::ConditionVariable#wait_until	nil	body is an until loop expression
MonitorMixin::ConditionVariable#wait_while	nil	body is a while loop expression
ERB#run	nil	print self.result(b) — print returns nil
ERB#initialize	void	constructor
ERB#def_method	Symbol	module_eval { eval("def …\nend") } returns the method name
ERB::DefMethod.def_erb_method	Symbol	delegates to ERB#def_method

Out of scope

• ObjectSpace.internal_super_of — although the doc says it returns "Class or Module", it actually returns the internal super entry, which can be a T_ICLASS object. That is not a real Module, so untyped is the correct type (the stdlib_test for it confirms this). Left unchanged.
• The core/class.rbs (allocate, new) and core/struct.rbs (self.new) untyped → instance proposals were left out: Class#new is special-cased by the type checker, and Struct.new(:a, :b) returns a new struct class (not an instance), so instance would be incorrect there.

Test plan

• bundle exec rbs -r stringio -r zlib -r monitor -r erb validate
• bundle exec rake stdlib_test
• bundle exec rubocop on the changed files