docs(failures): consolidated 5-mode failure taxonomy

Consolidates failure modes scattered across V2_DEEP_ANALYSIS,
E1_E2_RESULTS, E3_FINDINGS, KVC_EVICTION_GRANULARITY,
REAL_ALI_KVC_EXPERIMENT into a single lookup table with
five fields per mode: symptom → root cause → trigger →
current mitigation → real fix.

Five modes covered:
  A. Mooncake "instance not alive" cascade
     — E2 80%-failure pathology; admission no-space →
       seed burst → heartbeat drop → batch abort
  B. Cold-D / overlap-pinning
     — shared boilerplate hash pins all sessions to a
       subset of D's; load_floor_bonus is a patch, the
       real fix is exclusive_overlap redefinition
  C. Evict storm (session-level eviction)
     — release_session frees 38–88K tokens in one shot;
       fix is BLOCK_LEVEL_EVICTION_DESIGN
  C'. Reseed storm (turn-1 concurrent seeds)
     — startup-phase mooncake burst; fix is per-D
       pending-seed budget, frequency drops after C
  D. Streaming-session correction invariant crash (E3)
     — schedule_batch.py:1646 landmine, hotfixed by
       986f351, root-fix is removing the correction
       path entirely (BLOCK_LEVEL_EVICTION §3.7)

Each mode has a forensic link back to the original
experiment doc that surfaced it.

§6 adds a diagnostic cheat sheet: "if you see X, look at Y."
§7 wires every mode to a roadmap item — Milestone 1 should
graduate §1–§4 to "mitigated" and eliminate §5.

INDEX_ZH gets a new §1.6 section linking this and the
SGLang patch inventory.

No code change. Reading dependency for anyone debugging
a sweep or writing paper §Limitations.

docs/INDEX_ZH.md

@@ -58,7 +58,14 @@
 | [REFACTOR_PLAN_V1_ZH.md](REFACTOR_PLAN_V1_ZH.md) | 为什么从 ts=10 切到 ts=1 |
 | [TEAM_REPORT_AGENTIC_PD_HYBRID_ZH.md](TEAM_REPORT_AGENTIC_PD_HYBRID_ZH.md) | ts=10 时代的结构性问题清单（多数已 supersede） |
 
-### 1.6 环境
+### 1.6 工程债 / 失败模式
+
+| 文档 | 内容 |
+|---|---|
+| [SGLANG_PATCH_INVENTORY_ZH.md](SGLANG_PATCH_INVENTORY_ZH.md) | 785 行 vendored SGLang patch 的归类清单（MUST-HAVE / WORKAROUND / EXPERIMENTAL / INSTRUMENTATION）—— 本分支新增 |
+| [FAILURE_MODES_ZH.md](FAILURE_MODES_ZH.md) | 5 类失败模式的诊断 + 缓解 + 真正修复（mooncake cascade / cold-D / evict storm / reseed storm / E3 invariant）—— 本分支新增 |
+
+### 1.7 环境
 
 | 文档 | 内容 |
 |---|---|