Function auto_sparse_should_enable 

pub fn auto_sparse_should_enable() -> bool

Combined decision: Phase 2j auto mode is enabled AND the detected language supports auto-enabling the Phase 2e sparse re-ranker.

This intentionally differs from auto_hint_should_enable() after the §8.15 / §8.16 / §8.17 JS/TS follow-up arc: embedding hints stay auto-on for JS/TS, but sparse weighting does not.