Normalize served prev_action_chunk + reserved-key sample_kwargs passthrough

[jiabinq]

Two related fixes to served Policy.infer. Both are backward-compatible.

Summary

1. RTC prev_action_chunk is normalized to model space before reaching sample_actions.

Pi0RTC.sample_actions(...) consumes prev_action_chunk in model space (post-Normalize), but Policy.infer was forwarding obs["prev_action_chunk"] from the wire raw. The Agilex inference clients under train_deploy_alignment/inference/agilex/ send a raw deploy-space slice of their execution buffer, so the guidance term operates on un-normalized inputs — a silent train-deploy contract break. The bug is masked because Agilex action norm-stats are close to unit-variance, so the magnitude error in the guidance term is small, but the contract is broken.

The fix adds _normalize_and_pad_prev_chunk that delegates to the same transforms.Normalize instance the serving pipeline uses (so use_quantile_norm is honored) and pads to the model's action_horizon. Wired from the loaded checkpoint via three new optional Policy constructor params (norm_stats, use_quantile_norm, action_horizon); policy_config.create_trained_policy sets them automatically — call sites don't change.

Also guards against silent d=0 cheap-path activation when a client sends prev_action_chunk without inference_delay (would otherwise run the eager loop with no prefix conditioning).

2. Reserved-key obs["_sample_kwargs"] allowlist for transport-layer sample_kwargs overrides (currently: noise).

Policy.infer(obs, *, noise=...) accepts noise for in-process callers, but the websocket protocol drops it — WebsocketPolicyServer._handler calls self._policy.infer(obs) only. This makes deterministic evaluation of a served checkpoint impossible.

The fix extracts an optional obs["_sample_kwargs"]["noise"] into the noise kwarg path. The reserved-key namespace (leading underscore) avoids collision with any future model that legitimately uses an observation field named noise. The explicit noise= kwarg takes precedence over the obs-supplied noise (in-process callers behave unchanged).

Backward compatibility

• Callers that don't include _sample_kwargs see no behavior change.
• Non-RTC Policy(...) callers see no behavior change — the new constructor params are optional with safe defaults.
• Existing RTC clients that were sending raw deploy-space prev_action_chunk will now receive correctly-normalized chunks. This is the bug fix. No client API change required.

Test plan

• Send the same observation twice with obs["_sample_kwargs"]["noise"] = <fixed> and verify the returned actions are bit-identical.
• Compare a served Policy.infer call with prev_action_chunk against a direct Normalize({"actions": stats})({"actions": raw})["actions"] and confirm the values forwarded into sample_kwargs match.
• Confirm an existing non-RTC checkpoint still serves correctly with no client change (smoke).

Audit context

Found during a downstream parity audit of OpenDriveLab/kai0 9d93078 deploy stack. Audit deliverables (downstream fork): notes/awbc_inference_dagger_parity_gaps.md, reference/awbc_inference_dagger_upstream_review.md.