Add opt-in undefined global via require "literal/kernel"

Requiring "literal/kernel" defines `Kernel#undefined`, so a bare
`undefined` returns `Literal::Undefined` anywhere in the application.
It is opt-in because the trade-off — a typo'd bare `undefined` stops
raising `NameError` and starts flowing a truthy sentinel — belongs to
the application, not the library. Literal never loads it internally,
which subprocess tests enforce.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: joeldrapper

lib/literal.rb

@@ -7,6 +7,7 @@ module Literal
 	OBJECT_ID = BasicObject.instance_method(:__id__)
 
 	Loader = Zeitwerk::Loader.for_gem.tap do |loader|
+		loader.ignore("#{__dir__}/literal/kernel.rb")
 		loader.ignore("#{__dir__}/literal/rails")
 		loader.ignore("#{__dir__}/literal/railtie.rb")
 		loader.ignore("#{__dir__}/ruby_lsp")

lib/literal/kernel.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+require "literal"
+
+# Opt-in global sugar, loaded with `require "literal/kernel"`: a bare
+# `undefined` anywhere in the application returns `Literal::Undefined`.
+#
+# Literal never loads this itself — a typo'd bare `undefined` would otherwise
+# stop raising NameError and start flowing a truthy sentinel through the
+# program, and that trade-off belongs to the application, not the library.
+module Kernel
+	private def undefined
+		Literal::Undefined
+	end
+end

test/kernel.test.rb

@@ -0,0 +1,25 @@
+# frozen_string_literal: true
+
+# These assertions run in subprocesses: requiring "literal/kernel" in the test
+# process would define Kernel#undefined for the whole suite, masking any
+# accidental internal dependency on the opt-in global.
+
+LIB = File.expand_path("../lib", __dir__)
+
+test "requiring literal does not define a global undefined" do
+	assert system(
+		RbConfig.ruby, "-I", LIB, "-r", "literal",
+		"-e", "exit !Object.private_method_defined?(:undefined)"
+	) do
+		"Expected requiring literal alone not to define Kernel#undefined."
+	end
+end
+
+test "requiring literal/kernel defines a global undefined" do
+	assert system(
+		RbConfig.ruby, "-I", LIB, "-r", "literal/kernel",
+		"-e", "exit undefined.equal?(Literal::Undefined)"
+	) do
+		"Expected requiring literal/kernel to make a bare `undefined` return Literal::Undefined."
+	end
+end