The shared-KV path now keeps more CP metadata on-device and reuses
physical out-cache locations across MLA and NSA index writes, so each
layer avoids repeating logical-to-physical remaps. The in-seq CP
all-gather rerange path now delegates to tai-kernel when available and
falls back to the existing torch split/cat path with an explicit log.
This also extends the Phase8 prefetch machinery to cover shared KV
materialization metadata and keeps debug/fallback behavior gated so the
fast path is not polluted by diagnostic checks.
Constraint: Custom CP kernels must live in tai-kernel and be imported lazily from SGLang
Constraint: Decode does not use CP; these changes target NSA prefill CP in-seq-split shared KV
Rejected: Recompute physical local cache locations separately for MLA and index writes | repeats the same remap work every layer
Rejected: Keep the in-seq rerange Triton code inline in SGLang | duplicates kernel ownership and blocks tai-kernel reuse
Confidence: medium
Scope-risk: moderate
Directive: Keep CP collective ordering identical across ranks; do not add rank-local fallback decisions inside shared KV materialize paths
Tested: Remote g0034 container py_compile for modified SGLang/tai-kernel files; remote pytest test/registered/unit/layers/test_nsa_cp_utils.py passed with 24 tests
Not-tested: Full multi-node GLM5 prefill/decode throughput after the final commit boundary
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