Add flow inheritance for subclasses

[saturnflyer]

Summary

• Subclasses of Circulator classes now transparently inherit parent flows
• Declaring flow on a subclass for an attribute already defined on a parent raises ArgumentError recommending Circulator.extension
• Circulator.extension(ChildClass, :attribute) copies the parent flow, applies the extension, and stores it on the child class without affecting the parent
• Early extensions (registered before the child class exists) are applied lazily via an inherited hook
• Flow methods use dynamic transition lookup to ensure extensions work correctly on the very first call

Test plan

• Subclass inherits parent flows transparently (action methods, available_flows, state predicates)
• Redeclaring a parent's flow attribute raises ArgumentError with guidance
• Subclass can declare flow for a different attribute without error
• Late extension on subclass copies parent flow and applies changes
• Extension can modify existing transitions without affecting parent
• Early extension applied when subclass is defined
• Early extension modifying existing transitions works on first call
• Grandchild inheritance through intermediate class
• Sibling subclasses with different extensions don't interfere
• Standalone classes unaffected
• Deep copy isolation (mutating child flow doesn't affect parent)
• Full regression suite: 373 tests, 0 failures

🤖 Generated with Claude Code