CP shared-KV bs>1 compose must not silently fall back to dense full-buffer collectives when CUDA TAI materialize is expected. The fallback masks both correctness contract drift and severe synchronization/communication regressions, especially while comparing the symm path with the older IPC path.\n\nThis keeps CPU/unit-test fallback available, but makes production CUDA+TAI runs raise an explicit compose_v2 fail-fast for token-KV and index dense fallback. It also records the symm-vs-IPC comparison contract so barrier and collective counts are evaluated alongside elapsed time.\n\nConstraint: Production cache-hit-heavy bs>1 paths must expose unexpected dense collectives instead of silently taking them.\nRejected: Cherry-pick the old IPC branch wholesale | it conflicts with the symm compose design and would mix two transport protocols before benchmarking.\nRejected: Allow dense fallback with warning only | warning can be missed and still corrupts performance conclusions.\nConfidence: medium\nScope-risk: moderate\nDirective: Do not re-enable dense full fallback in CUDA+TAI compose paths without a benchmark proving it is intentional and a correctness test covering cache-hit bs>1.\nTested: python -m py_compile for cp_shared_kv_runtime.py and test_cp_shared_kv_runtime.py; git diff --check.\nNot-tested: Remote container pytest/ETE; local pytest is not reliable in this workspace because dependencies such as orjson are missing.
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