refactor: remove dense-mask fallback from sage_sparse_bf16
sage_sparse_bf16 now uses the native XPU sparse kernel exclusively. The
dense-mask fallback (rebuild the full block map, then run per-head dense
sagev1) is removed: it materialized a [B, Hq, Sq, Skv] mask and OOM'd at
long sequences, and the native kernel now covers every supported config.
Unsupported configs raise instead of silently degrading to the slow path.
