deepgemm: only Mega MoE cannot apply a post-expert norm
The guard refused a per-expert output norm on every DeepGEMM arm, but only Mega MoE
has to: it fuses the routing-weighted reduce into its kernel, so there is no seam for
the norm. The other two arms reduce in `_combine_routed_output`, so the norm rides the
rows just before it — the same place the reference forwards apply it.
`deepgemm_experts_guards` gains `post_expert_norm`, which only the Mega MoE arm sets
False. A model naming a norm — Muse-Spark names `input_scaled_rms_norm` — was locked
out of every DeepGEMM path and now loses only Mega MoE.
The old behaviour had no test, which is why relaxing it broke nothing; the new one
covers both halves so the Mega MoE restriction cannot silently disappear either.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
