[gg-rebased] perf(nf3): integrate Grid188 hybrid decode

[voipmonitor]

Summary

Forward-port the vLLM side of the mapped Grid188 NF3 hybrid decode path from #113 onto dev/gilded-gnosis-rebase at 533b037f35.

The two commits retain:

• exact-geometry fail-closed admission for the TP4 64-NVFP4 + 192-NF3 hybrid;
• the nvfp4_nf3_hybrid online quantization integration;
• VLLM_NF3_GRID188_DECODE as the explicit runtime gate;
• the per-step host-synchronization removal.

Requires local-inference-lab/b12x#36 (bc85ef3619) until that PR is merged.

Measured contribution

Matched TP4/DCP4/MTP3 C1 decode on the same GPUs and current v18 runtime:

Path	Median server decode
Grid188 disabled	95.34 tok/s
Grid188 enabled	106.28 tok/s
Incremental gain	+11.47%

Port validation

• ruff check: pass
• ruff format --check: pass
• git diff --check: pass
• Integrated CUDA/E2E validation is tracked with the v19 image.

Supersedes #113 for the upstream-rebased GG line.

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Review details

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Configuration used: Organization UI

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📥 Commits

Reviewing files that changed from the base of the PR and between 533b037 and dafc07c.

📒 Files selected for processing (7)

• tests/quantization/test_nvfp4_nf3_hybrid.py
• tests/v1/attention/test_indexer_dcp_localize.py
• vllm/config/quantization.py
• vllm/envs.py
• vllm/model_executor/layers/quantization/nvfp4_nf3_hybrid.py
• vllm/v1/attention/backends/utils.py
• vllm/v1/worker/gpu/spec_decode/autoregressive/speculator.py

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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