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.

tests/unit/test_health_selector.py

@@ -72,3 +72,43 @@ class TestHealthAwareSelector:
     def test_missing_stats_defaults_to_zero_inflight(self):
         sel = HealthAwareSelector(rng=lambda: 0.0)
         assert [s.name for s in sel.order([_src("s1")], {})] == ["s1"]
+
+
+class TestAdaptivePacer:
+    """AIMD 自适应并发(M2.5 设计 §3.35): 429 乘性削减,成功加性增长。"""
+
+    def test_initial_and_bounds(self):
+        from polygateway.sources import AdaptivePacer
+
+        pacer = AdaptivePacer(ceiling=32.0)
+        assert pacer.limit("s1") == pytest.approx(8.0)
+        for _ in range(200):
+            pacer.on_backpressure("s1")
+        assert pacer.limit("s1") == pytest.approx(1.0)  # 下限 1
+        for _ in range(2000):
+            pacer.on_success("s1")
+        assert pacer.limit("s1") == pytest.approx(32.0)  # 上限 = ceiling
+
+    def test_cut_and_growth_math(self):
+        from polygateway.sources import AdaptivePacer
+
+        pacer = AdaptivePacer(ceiling=32.0)
+        pacer.on_backpressure("s1")
+        assert pacer.limit("s1") == pytest.approx(8.0 * 0.7)
+        before = pacer.limit("s1")
+        pacer.on_success("s1")
+        assert pacer.limit("s1") == pytest.approx(before + 1.0 / before)
+
+    def test_inflight_gate(self):
+        from polygateway.sources import AdaptivePacer
+
+        pacer = AdaptivePacer(ceiling=32.0)
+        for _ in range(200):
+            pacer.on_backpressure("s1")  # limit → 1
+        assert pacer.admit("s1") is True
+        pacer.enter("s1")
+        assert pacer.admit("s1") is False  # 在途 1 ≥ limit 1
+        pacer.leave("s1")
+        assert pacer.admit("s1") is True
+        pacer.leave("s1")  # 多余 leave 不下穿 0
+        assert pacer._inflight.get("s1", 0) == 0