Add standard React controlled/uncontrolled component pattern to AbacusReact:
- Add `defaultValue` prop to support uncontrolled mode (component owns state)
- When `value` is provided, component operates in controlled mode (syncs to prop)
- When only `defaultValue` is provided, component operates in uncontrolled mode
- Update HelpAbacus to use defaultValue for interactive help
This enables interactive abacus in help mode where the component tracks its own
state while parent monitors via onValueChange callback.
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Co-Authored-By: Claude <noreply@anthropic.com>
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