feat: expose bare generation time and hedge flags in CallStats

CallStats gains hedges/generation_ms/hedge_won (all defaulted, appended
after total_latency_ms); _CallContext counts them via record_generation
(overwrite for chat/OCR, accumulate for embedding batches) and
register_hedge, and snapshot carries them out. All three _attempt
implementations time only the transport call itself on the same injected
clock as total_latency_ms; the chat sink is recorded by the orchestrator
so hedge winner attribution stays with T3. Hedge counters stay 0/False
until the T3 orchestration lands.

Red-green evidence: tests/outputs/137/t2/ (10 new tests AttributeError
red, then green; full unit+contracts 1621 passed).

tests/unit/test_embedding.py

@@ -319,6 +319,20 @@ class TestEmbedBatching:
         assert resp.cost is None
 
 
+class TestEmbedGenerationMs:
+    """裸生成时间按批累加(1.3.7 H8): generation_ms = 各批 transport 耗时之和。"""
+
+    async def test_generation_ms_sums_batch_transports(self):
+        # 0.25s 为二进制可精确表示值: int 截断下非精确值会因浮点误差少 1ms
+        clock = FakeClock()
+        transport = _ClockAdvancingEmbedTransport([(0.25, "ok"), (0.25, "ok")], clock)
+        client, _ = _embed_client([_src()], [], transport=transport, now=clock)
+        resp = await client.embed(["a", "bb", "ccc", "dddd"])
+        assert resp.call_stats is not None
+        assert resp.call_stats.attempts == 2
+        assert resp.call_stats.generation_ms == 500
+
+
 class TestEmbedPostProcess:
     async def test_normalize_l2(self):
         raw = EmbeddingTransportResult(