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/config.py

@@ -332,7 +332,7 @@ def _load_cache_keys(
 
 def _load_structured_retries(env: Mapping[str, str]) -> int:
     found = _first(env, "PGW_STRUCTURED_MAX_RETRIES")
-    value = int(_cast(found[1], "int", found[0])) if found else 1
+    value = int(_cast(found[1], "int", found[0])) if found else 2  # M2.5 迭代4: 1→2
     if value < 0:
         raise ValueError("PGW_STRUCTURED_MAX_RETRIES 不能为负")
     return value