feat: export discriminant match predicate

[okyrychenko-dev]

Summary

Expose the discriminant-matching predicate used by useEffectWhenMatch so it can compose with createEffectWhen.

Changes

• Rename the internal matching guard to the public matchPredicate factory.
• Export matchPredicate from the feature module and package root.
• Reimplement useEffectWhenMatch as a thin wrapper around matchPredicate.
• Add an end-to-end reusable-hook test covering runtime behavior and union narrowing.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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