feat: add AIMD adaptive concurrency pacing per source

P6 round 2 showed routing convergence turns account-level 429s into
the binding constraint (62% throttle rate at full concurrency). Each
source now carries a local AIMD limit: multiplicative cut on 429,
additive growth on success. Over-limit picks queue via the existing
quota-wait poll instead of burning retry budget or tripping the
circuit-open verdict.

src/polygateway/errors.py

@@ -8,7 +8,15 @@ SCOPE_REASONS = frozenset(
     {"circuit_open", "retry_exhausted", "stalled", "quota_exhausted", "no_sources"}
 )
 SOURCE_REASONS = frozenset(
-    {"network_error", "timeout", "rate_limited", "source_dead", "circuit_open", "cooldown"}
+    {
+        "network_error",
+        "timeout",
+        "rate_limited",
+        "source_dead",
+        "circuit_open",
+        "cooldown",
+        "adaptive_paced",  # M2.5 §3.35: AIMD 超限排队(quota-wait 通道)
+    }
 )