Stop dropping union members a module guard could match

[apiology]

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

Problem: When a guard tests for a module, a union member not already known to include it is dropped — so the inferred type omits a value that can actually occur.

module M; end
class A; end
class B; end
class A_with_M < A; include M; end
class B_with_M < B; include M; end

# @param x [A, B_with_M]
# @return [nil]
def f(x)
  return x if x.is_a?(M)

  nil
end
# Declared return type nil does not match inferred type ::B_with_M, nil for #f

A_with_M satisfies the declared A, passes the guard, and is returned, yet the inferred type admits only B_with_M. The mirror case — a module member against a class guard — is quieter still: every member drops, the empty result falls back to undefined, and the file reports no problems at all.

Solution: Keep the guard's type when the two types are related in neither direction and either side is a module, since a subclass can always mix a module in. Two unrelated classes remain disjoint under single inheritance and are still dropped.

The exact answer is their intersection, which cannot be spelled yet; a pending spec asserts it against castwide#1231.

Test plan:

• Strong self-typecheck against an origin/master baseline: 531 problems in 90 of 250 files on both sides, problem lists identical after normalizing paths and line numbers.

[apiology]

Claude: Reopened upstream as castwide#1361 with the same branch, title and description. Closing this one so the change has a single home.