feat: wire MXFP8/NVFP4 quantization into inference CLI

[lfengad]

Summary

Ports the inference-CLI plumbing from imaginaire4 MR 10201 (Inference optimizations [4/n]: MXFP8 / NVFP4 PTQ) that was missing on this side.

The quantization backend was already synced (via the 2026-07-10 release): QuantizationConfig (configs/base/defaults/quantization.py), apply_quantization_inplace (utils/generator/quantization.py), and the model_loader.py hook. But the user-facing OmniInference entrypoint was never wired, so --quantization-method had no effect. This PR connects them.

The missing pieces mapped exactly to MR 10201's packages/cosmos3/cosmos3/* files (→ cosmos_framework/inference/*); the projects/cosmos3/* files were already in.

Changes

• inference/common/args.py — add QuantizationMethod / QuantizationArgs / QuantizationOverrides; mix them into SetupArgs / SetupOverrides. Fields flow through build_setup's model_dump → model_validate (same as guardrails), so no extra build call is needed.
• inference/model.py — add Cosmos3OmniConfig.quantization property + setter and thread quantization_config through from_pretrained_dcp. Uses the local direct-construction style unstructure_config(QuantizationConfig(**value)) rather than upstream's LazyCall wrapper, matching the sibling parallelism/compile setters in this repo.
• inference/inference.py — add _get_quantization_config and pass it into both _create branches (load_model_from_checkpoint and from_pretrained_dcp).

Effect

• Standard experiment DCP path (load_model_from_checkpoint): quantization applied end-to-end via apply_quantization_inplace. ✅
• HF from_pretrained_dcp path: stores config.quantization without an apply hook — matches upstream MR behavior (not a regression introduced here).
• Only supported on Blackwell + replicated inference (no FSDP sharding); guarded by the existing model_loader.py DP-shard check.

Not included

• websocket_policy_server/serve_policy.py + README from the MR — that action-eval subtree is not present in this repo.
• The MR's cosmetic blank-line change in packages/cosmos3/cosmos3/args.py.

Verification

• py_compile passes on all three files.
• Load-bearing pydantic logic verified in-container (isolated replica): defaults, mutable-list-default isolation, build_quantization, Literal rejection of invalid methods, MRO/inheritance, and SetupOverrides → SetupArgs field flow — all assertions pass.
• Full module import not exercised: cosmos-framework requires Python ≥3.12 while the available i4 container env is 3.10; needs the cosmos-framework 3.13 venv to import the real module.

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