Prevent batched CP draft from silently leaving the target path

EAGLE draft shared-KV is supposed to mirror the target CP layout, so bs>1 must not fall back to legacy full-input or padded-hidden behavior when required batch metadata is missing or inconsistent. This change keeps the existing bs=1 compatibility path but makes batched CP draft fail fast on missing/mismatched spec hidden states, embedding pad metadata, or input embed shapes. The docs record the current W7 boundary: draft prefill follows target metadata, while scheduler admission and ETE remain gated.

Constraint: CP draft KV must mirror target layout and must not silently diverge under bs>1 shared-KV.

Rejected: Allow bs>1 to use the old full-input fallback | it can hide wrong owner/page metadata and corrupt accept length.

Confidence: medium

Scope-risk: moderate

Directive: Do not open the scheduler bs>1 CP gate until EAGLE accept length/output length are verified with this fail-fast path enabled.

Tested: Remote g0034 targeted EAGLE fail-fast unit test passed; remote full test/registered/unit/layers/test_nsa_cp_utils.py passed 70 tests.

Not-tested: EAGLE bs>1 ETE, because scheduler CP bs>1 admission gate remains closed.

docs/advanced_features/nsa_prefill_cp_shared_kv_bs_gt1_implementation_plan_zh.md

@@ -557,6 +557,13 @@ cp_size=8
 3. draft 不生成自己的 owner plan，一切跟随 target。
 4. target bs>1 ETE 通过后，再打开 draft local path。
 
+当前实现状态：
+
+- batch-aware input/position/spec-hidden split 已接到 draft local path；valid CP-local hidden 直接使用，full hidden 走 batch-aware split。
+- per-request last-token collect 已在 `cp_collect_last_token_hidden()` 中覆盖 batch plan。
+- 为避免打开 gate 后退回不正确的 full/draft fallback，bs>1 + `SGLANG_CP_DRAFT_SHARED_KV=1` + CP shared-KV 时，draft local path 对 missing/mismatch spec hidden、embedding pad metadata、input embeds shape mismatch 均 fail-fast。
+- admission gate 仍保持关闭；需要 EAGLE enabled ETE 验证 accept length/output length 后再打开。
+
 测试重点：
 
 - `cp_collect_last_token_hidden()` 对 bs=2 返回两个 request 的 last hidden。

docs/advanced_features/nsa_prefill_cp_shared_kv_bs_gt1_parallel_workstreams_zh.md

@@ -555,6 +555,18 @@ target path 正确后，再恢复 EAGLE/draft，并做远端 ETE/perf 验证。
 4. `cp_collect_last_token_hidden()` 返回每个 request 的 last hidden。
 5. EAGLE accept length 不能因为 cache hit/bs>1 掉到 1。
 
+### 当前状态
+
+- target 侧 batch metadata、batch split helper、per-request last-token collect 已有单测覆盖。
+- EAGLE/draft prefill local path 复用上述 batch-aware split helper：
+  - `cp_split_and_rebuild_1d()` 处理 draft input ids；
+  - `cp_split_and_rebuild_position()` 处理 positions；
+  - `_get_cp_local_spec_hidden_states()` 接受已 CP-local 的 draft hidden，或对 full hidden 走 batch-aware split；
+  - `cp_collect_last_token_hidden()` 按 request owner/offset collect last hidden。
+- bs>1 时不允许 silent fallback：如果 CP draft shared-KV 已开启但 spec hidden、embedding pad metadata、input embeds 形状不满足 batch fast path，直接
+  `[CP_SHARED_KV_FAIL_FAST][draft_batch_gt1_*]` 报错。bs=1 兼容 fallback 暂时保留。
+- scheduler 的 bs>1 admission gate 仍未打开；打开前必须完成下面 ETE 场景，尤其是 EAGLE accept length 与 output len。
+
 ### ETE 验证场景
 
 远端：