chore: add shared component internals

[FIL1994]

Summary

• add focused class-name, event-handler, and merged-ref helpers for upcoming component families
• add a hydration-safe stable ID hook with explicit ID overrides
• migrate ControlledTab to the shared stable ID contract
• cover each internal helper with focused unit tests

Validation

• bun run ci
  • 107 unit tests
  • package and React 18/19 compatibility fixtures
  • Storybook build
  • dependency audit

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