Revert billion suffix logic change due to performance regression (#56)

* Initial plan

* Clarify billion suffix logic consistency fix in Vietnamese number reading

Co-authored-by: hckhanh <[email protected]>

* Revert to original logic: use positionFromRight >= 6 for performance

Co-authored-by: hckhanh <[email protected]>

---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: hckhanh <[email protected]>

Author: Copilot