Record the phased design for turning NSA index-topk sharing from a compute-only optimization into HiCache and PD-transfer savings. The plan deliberately separates metadata validation from allocation compaction so prefill/decode state-slot mismatches fail before any bandwidth reduction can corrupt cache contents.
Constraint: CP shared-KV cache corruption previously appeared only on cache-hit paths, so state-layer mapping must be explicit before compact transfer or allocation.
Rejected: Compact L1/L2 index buffers first | without state_layer_ids this can silently map different logical layers to the same transfer slot across prefill and decode.
Confidence: high
Scope-risk: narrow
Directive: Do not skip the state_layer_ids phase before compacting NSA index state buffers.
Tested: Documentation placeholder scan and referenced source-path existence check.
Not-tested: Runtime behavior; this commit is documentation only.
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