traces/README: clarify w600 is the session-start window, not span

The trace actually spans ~2912 s (~48.5 min): all 274 sessions START within
the 600 s --window-seconds window, but their later multi-turn requests (34%
of rows, inter-turn gaps up to ~700 s) extend well past t=600 s. Remove the
misleading "~600 s span".

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

traces/README.md

@@ -3,7 +3,7 @@
 ## `w600_r0.0015_st30.jsonl`
 
 The primary replay trace for the routing / connector experiments
-(1214 requests, 274 sessions, ~600 s span). One JSON object per request:
+(1214 requests, 274 sessions). One JSON object per request:
 
 ```json
 {"chat_id": 1237198, "parent_chat_id": -1, "timestamp": 0.0,
@@ -30,6 +30,13 @@ requests, 2 h) — not redistributable; only this anonymized sample is shipped.
 The filename encodes the sampling params: `w`=window-seconds, `r`=sample-ratio,
 `st`=max-single-turn-ratio.
 
+**`w600` is the 600 s window of session _start_ times, not the trace duration.**
+The sampler keeps every session whose *first* request falls in a 600 s window,
+then includes *all* of that session's turns. Because agentic sessions are
+long-lived multi-turn (inter-turn gaps up to ~700 s), the actual trace **spans
+~2912 s (~48.5 min)** even though all 274 sessions start within the first
+598 s; 34 % of requests are later turns occurring after t=600 s.
+
 Regenerate (requires the dash0 source):
 ```bash
 python scripts/sample_trace.py \