feat(stats): cache hit-rate, tokens/turn, and result trend over time

First slice of #1424 (per-agent /stats enrichments):

- Backend: add per-bucket result_counts to the stats Snapshot (mirrors
  model_counts), so result outcomes can be charted over time, not just
  as a window total.
- Frontend: two new summary chips — cache hit-rate % (cached input vs
  all input-side tokens) and avg tokens/turn — both derived from the
  existing per-bucket token sums. Plus a stacked result-trend chart so
  error / rate-limit / compaction spikes are visible across the window.

Hive-wide aggregate, cost estimate, and container resource load land in
follow-up PRs.

frontend/packages/agent/src/stats.html

@@ -35,6 +35,7 @@
     <div class="stats-card"><h3>top tools</h3><div class="chart-wrap"><canvas id="chart-tools"></canvas></div></div>
     <div class="stats-card"><h3>wake source mix</h3><div class="chart-wrap"><canvas id="chart-wake"></canvas></div></div>
     <div class="stats-card"><h3>result mix</h3><div class="chart-wrap"><canvas id="chart-result"></canvas></div></div>
+    <div class="stats-card wide"><h3>result trend per bucket — errors / rate-limits / compactions over time</h3><div class="chart-wrap"><canvas id="chart-result-trend"></canvas></div></div>
   </div>
 
   <!-- Chart.js is now bundled into stats.js by esbuild (npm dep