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).

src/polygateway/ocr.py

@@ -395,11 +395,16 @@ class OcrClient:
         # layout 的 POST + ZIP GET 在同一次 `_invoke` 内,故这里只登记 **1** 次
         context.register_attempt()
         try:
+            # 裸生成时间(1.3.7 H8): 计时只包 transport 调用本身(同 RetryMW 口径);
+            # layout 的 POST + ZIP GET 在同一次 `_invoke` 内,属同一段生成耗时
+            gen_started = self._now()
             result = await self._invoke(kind, image, source, call_id)
+            gen_ms = int((self._now() - gen_started) * 1000)
             await self._record_quietly(self._breaker.record_success(entry))
             await self._record_quietly(self._quota.mark_progress())
             self._feed_outcome(source.name, ok=True)
             latency_ms = int((self._now() - started) * 1000)
+            context.record_generation(gen_ms, accumulate=False)
             await self._emit(
                 kind,
                 operation,