feat: add opt-in mid-stream stall hedging for chat

HEDGE__STALL_AFTER_S triggers a cross-source hedge when the primary
leg stalls mid-stream (independent of HEDGE__AFTER_S); two-phase
hedge window with event recheck discipline; watchdog-killed primary
converges on the surviving hedge leg. STREAM__STALL_THRESHOLD_MS
(1000ms default) adjustable via env/settings/constructor. Assembly
guards: ValueError on never-opening windows, embedding/OCR scope
warning (B4). Batch D 14 tests red-to-green, 1688 unit+contracts.

src/polygateway/client.py

@@ -239,6 +239,8 @@ class GatewayClient:
         call_deadline_s: float | None = None,
         hedge_after_s: float | None = None,
         hedge_max_extra: int = 1,
+        hedge_stall_after_s: float | None = None,
+        stall_threshold_ms: int | None = None,
         now: Any = time.monotonic,
         sleep: Any = asyncio.sleep,
         rng: Any = random.random,
@@ -247,15 +249,25 @@ class GatewayClient:
         self._call_deadline_s = ensure_call_deadline(
             call_deadline_s, "GatewayClient(call_deadline_s=...)"
         )
-        # 对冲守卫与 GatewaySettings 共用同一份(issue #24 H4): 直接构造这条路
-        # 不经过 settings,值域/交叉守卫若只挂在 settings 上就会被它绕过
-        self._hedge_after_s = check_hedge_assembly(
+        # 对冲守卫与 GatewaySettings 共用同一份(issue #24 H4 / 1.3.8 #28): 直接构造这条路
+        # 不经过 settings,值域/交叉守卫若只挂在 settings 上就会被它绕过。
+        # 返回契约(1.3.8 审查 B2): tuple(归一化 after_s, 归一化 stall_after_s)
+        self._hedge_after_s, self._hedge_stall_after_s = check_hedge_assembly(
             hedge_after_s=hedge_after_s,
             hedge_max_extra=hedge_max_extra,
             sources=sources,
             call_deadline_s=self._call_deadline_s,
             origin="GatewayClient(hedge_after_s=...)",
+            stall_after_s=hedge_stall_after_s,
+            stall_origin="GatewayClient(hedge_stall_after_s=...)",
         )
+        # 沉默计量阈值分发(1.3.8 设计 §6): None = 保留各 SourceConfig 自带值
+        # (不 stomp 直构造调用方的 per-source 设置);非 None 经 replace 写进每个源
+        # (replace 触发 SourceConfig.__post_init__ 重跑,值域校验复用同一处)
+        if stall_threshold_ms is not None:
+            sources = [
+                dataclasses.replace(s, stall_threshold_ms=stall_threshold_ms) for s in sources
+            ]
         emitter = (
             TelemetryEmitter(telemetry, scope=scope, pricing=pricing, text_cap=text_cap)
             if telemetry is not None
@@ -280,6 +292,8 @@ class GatewayClient:
             emitter=emitter,
             # max_extra 不下传(issue #24 H5): v1 编排固定单路对冲,无消费者
             hedge_after_s=self._hedge_after_s,
+            # 中段沉默对冲阈值(1.3.8 #28): 独立于 hedge_after_s 生效,守卫归一化值
+            stall_after_s=self._hedge_stall_after_s,
             now=now,
             sleep=sleep,
             rng=rng,
@@ -524,6 +538,9 @@ class GatewayClient:
             call_deadline_s=settings.call_deadline_s,
             hedge_after_s=settings.hedge_after_s,
             hedge_max_extra=settings.hedge_max_extra,
+            hedge_stall_after_s=settings.hedge_stall_after_s,
+            # settings 值为权威,恒分发(1.3.8 §3.5): 含缺省 1000 在内
+            stall_threshold_ms=settings.stall_threshold_ms,
         )
         _mark_owned_components(client, limiter=limiter, breaker=breaker, telemetry=telemetry)
         client._owns_cache = cache is None  # 缓存后端可以是 None(backend=none),helper 会跳过