Mixed prefix/current MLA reuse depends on the page-slot prefetch compose path to preserve padded tail semantics. The compact materialize/current merge path can re-expose suffix slack as valid dense rows, so the backend now raises with an explicit fail-fast marker instead of silently falling back.
Constraint: Page-aligned CP shared KV contract requires suffix tail slack to remain invalid.
Rejected: Keep compact merge as a fallback | it has a different dense-row contract and can hide correctness bugs.
Confidence: high
Scope-risk: moderate
Directive: Do not reintroduce merge_materialized_and_current_kv on MLA partial-current reuse without a page-slot correctness proof.
Tested: Remote py_compile for nsa_backend.py and test_nsa_cp_utils.py in g0034 container.
Tested: Remote pytest test_nsa_cp_utils.py::TestNSAInSeqCPUtils::test_mla_partial_current_path_fails_fast_instead_of_compact_fallback.
Tested: Remote pytest test_nsa_cp_utils.py test_cp_shared_kv_layout.py test_cp_shared_kv_runtime.py: 124 passed, 5 warnings.
Not-tested: Live ETE traffic and CUDA kernel execution for this fail-fast path.
Co-authored-by: OmX <omx@oh-my-codex.dev>
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