Add two-axis presentation navigation

[theclaymethod]

What changed

• Reserve Left/Right for slide navigation and Up/Down for ordered click-in states.
• Traverse visible drill targets automatically and expose a custom state-navigation hook for progressive components.
• Fall through from Down to the next slide when no internal state remains.
• Re-export and document the public API, update the demo, and add regression coverage.

Why

Presenter-driven decks need separate axes for slides and click-ins without one-off keyboard handlers. The runtime now provides that contract while preserving sheets, drills, direct navigation, and browser history behavior.

Validation

• npm test — 90 passed
• npm run ve:check
• npm run ve:eval-presentation — 25 passed
• git diff --check
• Two independent small-model, read-only adversarial reviews; all findings addressed and the final review approved