CP shared-KV cache-hit batches should compose long prefix pages and short current pages through page-slot IPC instead of falling back to dense all_reduce. Wire the runtime and prefetch consume paths to the TAI current-staging helpers, fail fast when the configured CUDA fast path cannot run, and document the bs>1 cache-hit benchmark evidence.
Constraint: bs>1 prefill must preserve the page-slot contract across fp8/bf16 and zero-lane current tails.
Rejected: Silent all_reduce fallback | hides correctness and performance regressions in production.
Confidence: medium
Scope-risk: moderate
Directive: Any future fallback in CP shared-KV CUDA fast paths must be explicit warning/fail-fast and covered by runtime tests.
Tested: Local py_compile cp_shared_kv_runtime.py and cp_shared_kv_prefetch.py; remote PYTHONPATH=python pytest -q test/registered/unit/mem_cache/test_cp_shared_kv_runtime.py (144 passed, 21 warnings, 2 subtests passed); remote TAI IPC benchmark fp8 bs>1 cache-hit matrix recorded in docs.
Not-tested: Full ETE mixed replay after replacing all current collectives with IPC.
(cherry picked from commit 8aa3b4ce59e0ebef5da5b0d07499a5f1d9785997)
142e7a5a64