A1: replayer instrumentation for cross-process join

RequestMetrics gains absolute unix timestamps (t_dispatch_unix,
t_first_token_unix, t_finish_unix), the proxy_request_id, the chosen
endpoint URL, and the trace hash_ids. Replayer sends
X-Request-Id: <session_id>:<turn_id>:<chat_id>:<idx> so proxy
breakdown rows can be joined to metrics by exact key.

Required by Batch 0 (online sequentiality proof) and Batch 1 reuse
decomposition; existing metrics.jsonl couldn't establish either.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

replayer/metrics.py

@@ -28,6 +28,12 @@ class RequestMetrics:
     requested_output_tokens: int | None = None
     finish_reason: str | None = None
     error: str | None = None
+    t_dispatch_unix: float | None = None
+    t_first_token_unix: float | None = None
+    t_finish_unix: float | None = None
+    proxy_request_id: str | None = None
+    endpoint_url: str | None = None
+    trace_hash_ids: tuple[int, ...] = ()
 
 
 class IncrementalMetricSink: