feat: default two structured re-asks and sharper AIMD cut

Round 5 left three residual failure classes; ladder exhaustion (3.2%
of structured calls with a single re-ask) and 429 leakage (5.8%, AIMD
oscillating above the sustainable point) are addressable: re-ask
default goes 1 to 2 (conservative vs instructor's 3) and the AIMD cut
factor drops to 0.5.

src/polygateway/sources.py

@@ -95,7 +95,7 @@ class AdaptivePacer:
     """
 
     _INITIAL = 8.0
-    _CUT = 0.7
+    _CUT = 0.5  # M2.5 迭代4: 0.7→0.5,残漏 5.8% 的 429 证明在临界点上方震荡
     _FLOOR = 1.0
 
     def __init__(self, *, ceiling: float) -> None: