Support Waterfill with MegaMoE backend

[xutizhou]

Summary

• allow Waterfill to preserve an explicit MegaMoE backend instead of forcing DeepEP
• allow the MegaMoE env path to be selected before Waterfill fallback logic runs
• use a rank-local shared expert slot semantic helper for TopK, FusedMoE, and DeepSeek shared expert fusion call sites
• add server-args tests for explicit MegaMoE and MegaMoE env selection under Waterfill

Dependency

Depends on sgl-project#27349, which adds the shared expert fusion loading compatibility needed by the current DeepSeek-V4-Flash FP4 test path.

Testing

• python3 -m compileall -q python/sglang/srt/layers/moe/fused_moe_triton/layer.py python/sglang/srt/models/deepseek_v2.py python/sglang/srt/layers/moe/topk.py python/sglang/srt/layers/moe/utils.py python/sglang/srt/server_args.py
• B200 MMLU subset: pure MegaMoE accuracy 0.8509861213; Waterfill+MegaMoE accuracy 0.8531775018

[xutizhou]

Closing this fork-internal stacked PR. Reopening as an upstream PR from xutizhou:fix/megamoe-waterfill to sgl-project:main, depending on sgl-project#27349.