perf(host): raise the folded-history caps so turn mode covers more turns

The host kept only the newest 160 request records and 150 events, so turn
granularity (which collapses steps into turns) could not show a meaningful
number of turns. Raised to 500 requests / 400 events.

src/host.ts

@@ -443,8 +443,10 @@ function foldInto(st: FoldState, events: readonly SessionEvent[]): void {
     }
   }
   st.n = events.length
-  if (st.requests.length > 160) st.requests = st.requests.slice(-160)
-  if (st.events.length > 150) st.events = st.events.slice(-150)
+  // Keep a deep-enough history for both display granularities: turn mode
+  // collapses steps, so these bounds must cover many turns, not just steps.
+  if (st.requests.length > 500) st.requests = st.requests.slice(-500)
+  if (st.events.length > 400) st.events = st.events.slice(-400)
 }
 
 function buildResult(st: FoldState): Snapshot {