[SM100] Select SiTU through the activation argument

[Fridge003]

Summary

• accept activation="situ" at the FP8xFP4 MegaMoE host/JIT kernel boundary
• thread an explicit compile-time SiTU flag into the SM100 kernel instead of inferring it from activation_clamp == 0.03125f
• preserve activation_clamp as a SwiGLU-only option and reject combining it with SiTU
• add a GB300 JIT/launch regression that distinguishes explicit SiTU from tightly clamped SwiGLU

This replaces the sentinel selection introduced by #67 while preserving the Kimi-K3 SiTU constants (beta=4.0, linear beta=25.0).

Testing

• ./build_sgl_deep_gemm.sh
• python sgl_deep_gemm/tests/test_mega_moe_situ.py --num-processes 1 (GB300)
• python sgl_deep_gemm/tests/test_mega_moe_l1_sentinel.py --num-processes 1 --num-tokens 32 (GB300, rel-RMSE 1.1648)
• Python compile checks, shell syntax check, Ruff, and diff checks