Add global Registry for cross-model enum field lookup

[kinnell]

Description

This PR serves to introduce an EnumFields::Registry that provides a centralized, globally-accessible index of all enum field definitions keyed by model name, enabling cross-model lookup and introspection of registered enum fields.

[Copilot]

Pull request overview

Introduces a globally accessible EnumFields::Registry to index enum field definitions by underscored model name, enabling cross-model lookup/introspection of registered enum fields.

Changes:

• Added EnumFields::Registry (a hash-like store with indifferent access) plus EnumFields.registry, EnumFields.register, and EnumFields.clear_registry!.
• Hooked EnumField#define! to automatically register every enum_field definition into the global registry.
• Added comprehensive specs for registry behavior and ensured test isolation by clearing the registry before each example.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
spec/spec_helper.rb	Clears the global registry before each spec to avoid cross-example state leakage.
spec/enum_fields/registry.spec.rb	Adds coverage for registry access, key normalization, integration with enum_field, and clearing behavior.
lib/enum_fields/registry.rb	Implements the new Registry store and #register behavior.
lib/enum_fields/enum_field.rb	Registers definitions into the global registry during define!.
lib/enum_fields.rb	Adds autoload + module-level registry helpers (registry, register, clear_registry!).

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