Stop dropping union members that differ only by type parameter

[apiology]

Problem: A method called on a variable typed as two instantiations of the same generic class returns only one of them — the rest of the union is silently dropped.

# @generic T
class Box
  # @return [generic<T>]
  def get; end
end

# @param b [Box<Integer>, Box<String>]
def one_order(b)
  b.get  # => Integer   (expected Integer, String)
end

# @param b [Box<String>, Box<Integer>]
def other_order(b)
  b.get  # => String    (expected Integer, String)
end

Every union of same-class generics is affected, no declaration order returns the full union, and the truncated result reads as a legitimate concrete type rather than an error.

Solution: Dedup the per-member method pins on both path and return_type.rooted_tags instead of path alone, using rooted_tags rather than tag because ComplexType defines no #tag and so falls through to its first union member.

Fixes #1272.

🤖 Generated with Claude Code