Add the GLM MoE MegaMoE fast path before the existing DeepEP/normal branch selection.
Keep the existing none+flashinfer_trtllm path as the miss path when MegaMoE weights, envs, or token caps do not satisfy should_use_mega_moe().
Match upstream MegaMoE shared-expert TP1 semantics for the separate shared expert path.
Constraint: do not change the baseline none+flashinfer_trtllm dispatch path.
Feature-flag: --moe-a2a-backend=megamoe.
Conflict-hotspots: python/sglang/srt/models/glm4_moe.py.
Scope-risk: shared-expert TP1 behavior matters when shared expert fusion is disabled.
Tested: PYTHONPYCACHEPREFIX=/private/tmp/sglang_pycache python3 -m py_compile python/sglang/srt/models/glm4_moe.py.
Tested: git diff --check.
Not-tested: GLM 5.2 MegaMoE GPU e2e; local environment lacks target runtime and hardware.
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