docs: publish which errors reach callers and which the library absorbs

TransientError and SourceDeadError read like caller-facing contracts in
the taxonomy table, but the retry loop catches both and repackages them as
AllSourcesExhausted, so they never arrive. That is only discoverable by
reading middleware/retry.py, and a downstream project wrote a whole design
section on the false premise before checking.

The new table states the split outright, including that
GovernanceBackendError now sits on the caller-facing side and
SourceNotConfiguredError deliberately does not join the retryable family.

research-wiki/migrations/chsanalyzer.md

@@ -181,7 +181,7 @@ stack = ExtractionProviderStack(
 | R4 | 六道闸+契约 5 条、服务器时钟窗口、settle 落 acquire 窗口、transient 按 est 保守结算 | M2 | §7.3 大体覆盖 |
 | R5 | RequestRejected 二分(真实响应记成功/本地拒绝释放探针);换源重试跨源计数口径 | M2 | 须进 M2 设计 |
 | R6 | OCR ZIP 协议 + bbox 数值防御下沉;OCR Usage=0;glm 白名单预留 | M3 | §7.10 已覆盖 |
-| **G1** | ✅ 已闭(M3 核实): 库 `GatewayUnavailableError` 一族自 M1 起携 `scope/reason/retry_after_s/per_source_reasons`(errors.py:74-105),chat/embedding/OCR 三循环抛出点均已填充且有契约测试钉住;项目侧仅剩约 10 行翻译 shim(库异常 → ProviderUnavailableError)或 tracking.py 直接 except 库异常 | M2 | 已闭 |
+| **G1** | ✅ 已闭(M3 核实): 库 `GatewayUnavailableError` 一族自 M1 起携 `scope/reason/retry_after_s/per_source_reasons`(errors.py:74-105),chat/embedding/OCR 三循环抛出点均已填充且有契约测试钉住;项目侧仅剩约 10 行翻译 shim(库异常 → ProviderUnavailableError)或 tracking.py 直接 except 库异常。**2026-08-06 补(issue #7,库 1.1.0)**: 治理后端故障(`GovernanceBackendError`,Redis 挂等 fail-closed 情形)此前**不在**该族内,`except GatewayUnavailableError` 接不住,会落进 `_TERMINAL` 兜底而消耗业务失败预算;现已归入该族(`reason=governance_backend_down`,`retry_after_s` 默认 5.0),tracking.py 一条 except 即覆盖完整,**无需为它单列分支**。同批新增的 `SourceNotConfiguredError`(源名与配置不匹配的装配缺陷)**有意在族外**,应当落进 `_TERMINAL` 让配置错误浮出水面 | M2 | 已闭 |
 | **G2** | ✅ 已闭(2026-07-30 核实): `est_tokens` 已进 ARCH §7.7 SourceConfig 字段清单,且 §7.3 `try_acquire` 的 est 来源已定义为 `SourceConfig.effective_est_tokens()`(显式值优先,否则按 `tpm // 60` 派生)。双职责一并拆开:该字段只剩 TPM 预扣的可选调优覆盖,usage 缺失不再由它兜底(见 §7 行 151 的推翻判定) | M2 | 已闭 |
 | **G3** | ⚠️ §4.3 层序图文矛盾:图示 熔断→限流→重试(重试最内),但理由要求"每次重试重新过限流闸"且熔断/限流是 per-source 的、选源在重试循环内(governance.py:120-167 实践为每次尝试执行 选源→冷却备忘→permit→熔断门)。洋葱不澄清"逐次准入"机制则多源语义无法成立 | M2 | **架构缺口**,澄清 §4.3/§4.4 |
 | **G4** | ⚠️ per-scope 韧性配置命名(`{SCOPE}__RETRY__*`/`BREAKER__*`/`BACKPRESSURE__*`/`SELECTOR`/`GLOBAL__*`)未进 ARCH §9,现文只有平铺 `LLM_*` 键;CHSAnalyzer 的 VLM/OCR 两 scope 参数各异,平铺键无法表达 | M2 | **架构缺口**,修订 §9 |