Fix NoMethodError on enum methods when subclass defines no enums of its own

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Summary

Fixes #49.

Ruby::Enum's class methods (keys, values, key?, value?, key, value, to_h, parse, each) read directly from the @_enum_hash/@_enums_by_value instance variables. These are only initialized the first time define is called on a class (or explicitly in the included hook).

A subclass that inherits Ruby::Enum behavior purely through Ruby class inheritance - without calling define itself and without re-including Ruby::Enum - never has these ivars set on its own singleton class, so any of the above methods raised NoMethodError: undefined method '...' for nil.

class PostMediaType
  include Ruby::Enum
  define :FOO, 'foo'
end

class PostVideoType < PostMediaType
end

PostVideoType.values
# NoMethodError: undefined method `values' for nil

Fix

Introduces private _enum_hash/_enums_by_value reader methods that lazily default to {}, and uses them everywhere instead of the raw instance variables. This preserves all existing behavior (including values' superclass-aware recursion) while making the other methods null-safe for subclasses with no enums of their own.

Testing

• Added a SubclassWithNoOwnDefines test class and specs covering values plus a regression test that keys, key?, value?, key, value, to_h, parse, and each no longer raise.
• bundle exec rspec spec/ - 55 examples, 0 failures, 100% line coverage.
• bundle exec rubocop - clean.
• spec_i18n suite - 3 examples, 0 failures.

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