[WS2][PR5][Attention] Add distributed CP drift benchmark artifacts

[inaniloquentee]

Summary

Implements PR5 of #235: the rank-aware WS2 Attention acceptance runner, report schema, and fail-closed validation for distributed forward, backward, communication, and repeatability evidence.

Gate

Lane	Role	Acceptance boundary
CP reference benchmark	Forward/backward attribution and rank-aware report generation	Validates required metrics and provenance; not production communication by itself
p2p_nccl_reference	Independent communication correctness reference	Fixed logical block order and FP32 (Out, LSE) merge; no strict training autograd claim
cuda_ag_rs --strict-shared-core	Required production operator gate	Same deterministic core for train/rollout, self-owned AG/RS autograd, Split-KV disabled, no native arithmetic/fallback
FlashInfer / TE / Megatron	Compatibility or diagnostic lanes	Never substitute for the strict shared-core gate
AllReduce	Optional diagnostic	Outside the Attention required gate

The validator rejects CPU/dry-run-only evidence, missing CUDA/NCCL, incomplete topology/manifests, fallback, non-FP32 merge state, wrong downcast point, missing gradients, or non-bitwise repeats.

Strict H100 results

GPUs	Topology	Replicas	Out / LSE / dQ / dK / dV	Repeat	Result
2	TP=1, CP=2	1	Bitwise; max abs 0.0	Out/LSE bitwise	Pass
4	TP=2, CP=2	1	Bitwise; max abs 0.0	Out/LSE bitwise	Pass
8	TP=2, CP=2	2	Bitwise; max abs 0.0	Out/LSE bitwise	Pass

Every rank reported global_failure_count=0, core_id=rlkernel.attention.deterministic_core.v1, fallback=false, native_attention_arithmetic=false, and communication_autograd=true.

Evidence

Artifact / check	Result
strict-shared-core-agrs-2rank-8309b77.json	Passed
strict-shared-core-agrs-tp2-cp2-4rank-8309b77.json	Passed
strict-shared-core-agrs-tp2-cp2-replica2-8rank-8309b77.json	Passed
Focused local validator suite	22 passed
Formatting, compile, and diff checks	Passed

Boundary

This PR accepts operator-level Attention evidence. It does not turn optional native TE/FlashInfer arithmetic, AllReduce, or an unfurnished Megatron/vLLM checkpoint run into a required result. The executable strict path is owned by #279; backward semantics are owned by #284. Related: #235, #294.

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