The runner was quantizing the padded_hidden (4096 rows) and then
taking x_sf[:num_slots] (first 48 rows). This only got scales for
expert 0 (the first 48 rows of the padded buffer), not the scales
for tokens scattered across padded positions (expert 1 at row 128, etc).
Fix: quantize slot_hidden (sorted tokens, num_slots rows) to get
correct per-token x_sf, then scatter x_fp4 into padded FP4 buffer
for the GEMM. The scale assembly now receives the correct x_sf.
Added hidden_fp4 and activated_fp4 padded buffers for FP4 scatter.