Add known bug for splat fallback with union types

When [*x] has x typed as Array[T] | Symbol, the Symbol side doesn't
get wrapped as [Symbol] because to_a succeeds for the Array side,
making @ary non-empty and skipping the fallback.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: mame

scenario/known-issues/splat-union-fallback.rb

@@ -0,0 +1,12 @@
+## update
+def foo(x)
+  [*x]
+end
+
+foo([:int])
+foo(:sym)
+
+## assert
+class Object
+  def foo: (Array[:int] | :sym) -> Array[:int | :sym]
+end