Fail fast when CP compose would hide dense fallback

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.

docs/advanced_features/nsa_prefill_cp_shared_kv_bs_gt1_l1_prefetch_zero_sm_plan_zh.md

@@ -734,3 +734,53 @@ P7 GSM8K/replay/Nsight 验证
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 不要先做 P6 再修 P2/P3。0SM kernel 只解决 transport 代价，不解决 bs>1 prefix/current slot 语义；如果语义仍是 scalar prefix，kernel 越快只会越快地产生错误。
+
+## 2026-06-12 追加：symm compose 与旧 IPC kernel 的对比口径
+
+`symm-syh` 分支的 compose/symm kernel 可能比旧 CUDA IPC current-staging kernel 单次更快，但不能只比较 kernel elapsed time。需要把同步点作为一等指标，否则可能出现“单 kernel 快，但每层/每 buffer 多一次同步，ETE 更慢”的误判。
+
+对比 benchmark / Nsight trace 必须至少拆分以下事件：
+
+1. **IPC capability agreement**
+   - `_agreed_tai_ipc_peer_ptrs()` 里的 group agreement / all-reduce。
+   - 应确认是每个 pool tensor 一次，还是每个 forward/layer 重复触发。
+
+2. **symm barrier**
+   - `cp_symm_barrier()` 调用次数、耗时、等待方差。
+   - 需要按 token KV / index buffer 分开统计。
+   - 当前 tai-kernel 实现是 1 个 block / 1 个 warp 的 CUDA spin barrier，
+     不是 copy-engine/0SM 路径；它占用很少 SM，但会在当前 stream 上形成
+     明确同步点。判断 symm 是否优于旧 IPC 时，必须把这个 barrier 的次数和
+     rank 间等待方差算进去。
+
+3. **symm mega gather**
+   - `materialize_cuda_ipc_peer_pages_slot_dense()` 在 symm combined ptr table 上的耗时。
+   - 统计 prefix pages、current pages、request 数、dense pages。
+
+4. **compact current reduce**
+   - symm 未开启或不可用时的 `_reduce_current_pages_compact()`。
+   - 这是 compact current collective，不是 dense full fallback；但仍会同步/占用通信资源，需要单独计数。
+
+5. **dense full fallback reduce**
+   - `_all_reduce_materialized_buffer(... v2_full ...)`。
+   - 生产 CUDA + TAI materialize 开启时不允许静默发生；应 fail-fast 暴露 `CP_SHARED_KV_FAIL_FAST][compose_v2]`。
+
+6. **CPU descriptor / plan 成本**
+   - `get_or_build_compose_plan()` cache hit/miss。
+   - per-forward 是否复用 descriptor；不要把一次性 build 成本误算到每层 steady state。
+
+建议 benchmark 矩阵：
+
+- dtype：bf16 / fp8_e4m3；
+- batch size：1, 2, 5, 10；
+- extend：1k, 2k, 10k, 40k, 65k；
+- cached/prefix：100k, 200k, 300k；
+- case：cache-hit partial-current、current-only、multi-request shared prefix；
+- mode：legacy dense fallback（只作为基线，不允许生产静默）、old CUDA IPC current staging、symm staging、symm prefetch。
+
+结论标准：保留默认路径必须同时满足：
+
+- 无 dense full fallback；
+- 同步点数量不高于旧 IPC 路径，或同步耗时被更少 kernel/更高带宽抵消；
+- ETE replay 在 cache-hit-heavy 短 extend 场景提升，而不是只在 micro benchmark 提升；
+- GSM8K cache-hit 二轮精度不掉点。