Record compact IPC staging capacity contract

The index current IPC startup failure came from a tai-kernel validation mismatch:
compact staging allocations are rounded capacity buffers and do not need to be
page-size divisible. Record the failure signature and the intended contract so
future changes do not reintroduce dense-sized or page-multiple assumptions.

Constraint: TAI compact staging fix lives in the kernel repository; this change documents the runtime contract in SGLang
Rejected: Leave the issue only in commit history | future debugging would repeat the log search and root-cause analysis
Confidence: high
Scope-risk: narrow
Directive: For compact current staging, validate capacity bytes, not page-array shape
Tested: Remote cjy-glm5-new SGLang runtime helper suite -> 144 passed, 2 subtests passed
Not-tested: Documentation rendering
(cherry picked from commit ce281c19be5d54cd2cb345543b0384d7560c95ef)

docs/advanced_features/nsa_prefill_cp_shared_kv_ipc_collective_replacement_plan_zh.md

@@ -320,9 +320,35 @@ peer read staging[compact_i] -> dense_current[dense_page_id_i]
 3. 不允许回到静默 all_reduce fallback。CUDA + TAI materialize 开启时，compact IPC 不可用仍应 fail-fast。
 4. 后续 fused current fill+publish 仍有价值：compact staging 解决容量问题，但 current 仍有一次额外 HBM copy。
 
+### 2026-06-12 index current 启动失败补充
+
+启动阶段又触发：
+
+```text
+[CP_SHARED_KV_FAIL_FAST][tai_ipc_materialize]
+reason=index_current_ipc_unavailable
+dense_shape=(2, 8448)
+```
+
+前置 warning：
+
+```text
+reason=current_paged_kernel_failed
+error=staging byte size must be divisible by page_nbytes
+```
+
+这是 compact publish op 的 C++ check 错误，不是 runtime descriptor 错误。index page bytes 为 8448，current staging cache 会按 power-of-two 扩容到 16384；16384 不能被 8448 整除，但这完全合法。compact staging 只要求：
+
+```text
+staging_nbytes >= num_slots * page_nbytes
+```
+
+不要求整个 staging allocation 被 `page_nbytes` 整除。TAI 单测已覆盖 `page_nbytes=8448, staging_nbytes=16384`。
+
 ### 验证
 
 - 新增 TAI CUDA 单测：dense 10 pages、staging 3 pages，只发布 `[1, 4, 9]` 到 compact staging，验证不需要 dense-sized staging。
+- 新增 TAI CUDA 单测：`page_nbytes=8448`、`staging_nbytes=16384`，验证 power-of-two staging capacity 可用于 index current page。
 - SGLang runtime 单测覆盖 token/index current helper：publish 使用 dense destination pages，peer materialize 使用 compact source pages。
 - `benchmark_cp_shared_kv_ipc_gather.py` 已同步改成 compact current staging 合同；quick smoke：
   `bs=2 cached=4096 extend=1024 fp8/uint8` 下 dense all_reduce p50 0.459ms，IPC compose p50 0.310ms。