Keep destroy() from tearing down another active tour

[MaciejZet]

Summary

Fixes #504. destroy() on one instance was removing the shared overlay, body classes, and highlighted state for a newer tour. A delayed destroy() from an old highlight could therefore close the tour that had replaced it.

This patch keeps ownership of the page-level tour explicit. A stale instance releases only its own listeners and highlight; the current instance keeps its popover, overlay, body classes, and active highlight. An instance that never started, or was already destroyed, returns without touching the document. The same instance can start another highlight after a real teardown.

This PR stays focused on the stale-instance destroy() invariant. setSteps() behavior is unchanged.

Test plan

• pnpm --dir packages/driver test:run (341 tests)
• pnpm build
• Second destroy() after teardown leaves the replacement tour up
• destroy() while the old instance still reports isActive() leaves the replacement tour up
• destroy() on an instance that never started leaves the live tour up
• The same instance can highlight again after destroy()
• An animated highlight destroyed across frames does not catch the next tour

Refs #504