Structurally verify RBS interface-typed expectations

[apiology]

🤖 Filed by Claude, not Vince — acting on his behalf via his GitHub credentials.

Problem

RBS 4.1 broke hash lookups in Solargraph.

# @param registry [Hash{Symbol => Array<String>}]
# @return [Integer]
def count(registry)
  # #count return type could not be inferred
  registry.fetch(:x).length
end

4.1 loosened Hash's key lookups from the class parameter K to the structural interface Hash::_Key. Solargraph has rough workarounds for a few existing interfaces and no general way to satisfy one, so Symbol fails to match the parameter, every #[] and #fetch overload is rejected as a non-match, and the caller is left with no usable type.

class Hash[unchecked out K, unchecked out V]
  # rbs 4.0.3
  def fetch: (K arg0) -> V

  # rbs 4.1.3 — no K anywhere in the signature
  def fetch: (_Key key) -> V

  interface _Key
    def hash: () -> Integer
    def eql?: (untyped rhs) -> boolish
  end
end

Solution

Match interfaces structurally: an inferred type conforms to an interface-typed expectation if its own method stack has every method the interface declares. Symbol has #hash and #eql?, so it satisfies Hash::_Key without anyone declaring an include.

:allow_unmatched_interface now applies only where the expected interface has no methods Solargraph can see, leaving nothing to check.

Limitation: generic interfaces (_Each[Elem]) are checked by method name only, not parameter or return types, since they get turned into YARD #duck_types.

Fixes #1232