feat: exempt 429 pushback from retry budget with stall ceiling

Round 6 hit account-level rate throttling the concurrency AIMD cannot
absorb: at 26 req/min the gateway still returned 16% 429s and each one
burned a third of the retry budget. Retry-After-guided 429s now back
off without consuming attempts (gRPC pushback semantics); the retry
loop gains a per-call ceiling using the same dual-condition stall
verdict as quota-wait (local window exceeded AND no global progress).

research-wiki/migrations/chsanalyzer.md

@@ -188,3 +188,5 @@ stack = ExtractionProviderStack(
 | 开路时长指数递增,`retry_after_s` 上限由 cooldown_s(60s)变为 max_cooldown_s(缺省 300s) | **G1 消费点注意**: tracking.py arq defer 延期时长最多放大 5 倍;属期望行为(反复坏的源就该等更久) |
 | 阈值自动抬升由全局并发改为源级并发 | CHS .env 注释约定的本意(防并发误熔)保留;全局并发抬升是 M2 错误移植,已废除 |
 | 缺省选源 round_robin → health_aware(EWMA×在途 P2C) | 显式配置 `LLM__SELECTOR=round_robin` 者行为不变;迁移时建议直接吃新缺省 |
+| 429 不消耗重试预算(pushback 语义),调用级时间上限 = stall_window_s | 饱和期调用延迟上限从 ~3×backoff 变为 stall_window(300s);CHS 快失败偏好者可调小 `LLM__BACKPRESSURE__STALL_WINDOW_S` |
+| 结构化重问缺省 1→2 | 解析失败时最多多一跳成本;`PGW_STRUCTURED_MAX_RETRIES=1` 可显式还原 |