Launching CP shared-KV prefetch from MLA prepare or the indexer can make
next-layer prefix work overlap current-layer MQA/materialization instead of
the attention window. Centralize the launch in the NSA backend after
current-layer materialization and before attention, and leave the early
indexer hook inert so the call site cannot regress silently.
The accompanying notes capture the draft-as-forward-layer follow-up plan and
the latest OOM diagnosis: the observed 178945-token failure matches the CP
in-seq MQA logits allocation, so the follow-up fix is q-dimension chunking in
_get_topk_ragged_with_cp(), not a max-prefetch-size gate.
Constraint: CP shared-KV prefetch must avoid overlapping current-layer MQA/materialization and must not add silent fallback behavior.
Rejected: Limit maximum prefetch size | hides the CP logits peak and can reduce cache/prefetch effectiveness.
Rejected: Keep prepare/indexer launch sites | they place next-layer collectives in the wrong overlap window.
Confidence: medium
Scope-risk: moderate
Directive: Do not reintroduce early CP prefetch launch without checking Nsight overlap and CP MQA memory peaks.
Tested: Local git diff --check and py_compile for touched Python files.
Tested: Remote container py_compile plus targeted pytest: 3 passed, 5 warnings.
Not-tested: Full ETE under production traffic after this commit.
Not-tested: CP in-seq MQA logits chunking; documented as follow-up.
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