feat: redefine generation_ms as net generation time with stall attribution

CallStats gains stall_ms/stall_count (None = no stall observation);
winner sample triple (wall, stall_ms, stall_count) converted once in
_attempt success branch; net = max(0, wall - stall) converged into the
shared _record_winner_sample used by both the single-path guard and
hedge winner registration. Two-fail hedge rounds record nothing
(no winner to attribute). Design section 4.2; issues #27.

tests/unit/test_hedge.py

@@ -569,6 +569,27 @@ class TestHedgeFailureCombination:
         # 两败轮次同样登记对冲路数(设计 §4.5: 实际并发发出即计)
         assert ctx.snapshot().hedges == 2
 
+    async def test_stall_attribution_none_when_no_winner(self):
+        """两败轮次无赢家(1.3.8 排除条款): generation 不记、stall 归因恒 None。
+
+        对冲已实际发出(hedges 照登),但无赢家路 → 裸生成时间与沉默归因都
+        无归属;两路各自的沉默归属各自 attempt 遥测行(那是 T3 的事)。
+        """
+        transport = HedgeTransport(
+            {
+                "s1": [("fail_after", 0.1, lambda: TransientError("p", source_name="s1"))],
+                "s2": [("fail_after", 0.12, lambda: TransientError("h", source_name="s2"))],
+            }
+        )
+        mw, _, _ = _harness([_src("s1"), _src("s2")], transport, max_attempts=1)
+        ctx = _ctx()
+        with pytest.raises(AllSourcesExhausted):
+            await asyncio.wait_for(mw(_req(ctx=ctx)), timeout=5)
+        stats = ctx.snapshot()
+        assert stats.hedges == 1 and stats.hedge_won is False
+        assert stats.generation_ms == 0
+        assert stats.stall_ms is None and stats.stall_count is None
+
     async def test_both_429_refund_no_budget(self):
         """两路皆 429: 免预算且耗时退 stall 账——小 stall 窗下终局 stalled 而非耗尽。"""