feat: magnitude branch forward/backward uses [h_s2; Q_dir] conditioning
Forward: branch 1 FC GEMM reads [B, SH2+3] from mag_concat_buf.
All 3 forward paths (online_raw, online_f32, target_raw) updated.
Concat uses previous step's direction logits (one-step lag, negligible).
Backward: branch 1 dW uses wider X, dX writes to d_mag_concat_buf.
strided_accumulate extracts first SH2 columns into d_h_s2 with beta=1.
d_Q_dir (last 3 columns) discarded — detached, no backprop to direction.
Fixes backward goff_b_b1fc offset: uses ah*(sh2+3) matching compute_param_sizes.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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