mag_concat now uses vsn_masked (feature-selected) instead of raw h_s2
for its first SH2 columns. strided_scatter kernel writes tight [B,SH2]
vsn_masked into wide [B,SH2+3] mag_concat preserving Q_dir in last 3.
launch_mag_concat → launch_mag_concat_from with explicit source ptr.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>