Stage 1 of the fused epilogue: applies SiLU (x * sigmoid(x)) to the
full accumulator register tensor before writing BF16 to C.
This validates that cute.exp and element-wise FP32 operations work
on CuTe register tensors in the epilogue. The gate/up pairing is
not yet implemented (Stage 2).
The fused_swiglu flag is const_expr(0) by default, so the standard
epilogue path is unchanged unless the flag is enabled.