Add gated GLM-5.2 bit-exact compatibility paths

[zrr1999]

Summary

• add opt-in accuracy-compatible DSA execution and backward behavior
• add opt-in RMSNorm formulation/backward compatibility and wire it through MTP specs
• add opt-in explicit fp32 router gating for deterministic GLM-5.2 alignment

All compatibility paths are gated; default Megatron behavior remains unchanged.

Validation

• focused unit tests and Ruff checks passed for the individual commits
• official-weight GLM-5.2 reduced-profile probes ran on NVIDIA H800 with CUDA/bfloat16
• forward step-0 loss matches the independent Paddle path at 12.416634559631348
• multiple named backward boundaries are bit-exact; selected embedding gradient, optimizer, and multi-step parity are not yet established

Status

Draft while strict cross-framework backward localization continues. This PR does not claim full-model or 100-step bit-exact completion.