Checkpoint keys are model.layers.N.shared_experts.gate_proj.weight
but model params are layers.N.ffn.shared_experts.gate_up_proj.weight.
The .ffn. was missing from the rename, so stacked gate_up_proj
never matched params_dict.
The stacking logic skipped any key containing '.experts.' to avoid
MoE routed expert weights. But 'shared_experts' also matches that
substring, so gate_proj and up_proj were never stacked into
gate_up_proj. Changed to '.ffn.experts.' which only matches the
routed experts path.
Also includes POST-LOAD all-zero param scan.
