fix: retain tab focus across fragment-wrapped selection changes

[feritcemkahraman]

When controlled tabs are wrapped in a fragment, changing selectedIndex leaves keyboard focus on the old, now unselected tab.

Check the registered tab elements for focus instead of checking only direct React children. This preserves focus across selection changes while leaving focus outside the tabs alone.

Validation: the focus regression failed before the fix; all 81 tests, full ESLint, and the CommonJS/ESM builds pass. Windows checkout line endings were normalized locally for lint. The real-browser matrix and documentation build were not run.