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.

hive-ag3nt/src/stats.rs

@@ -129,6 +129,10 @@ pub struct Bucket {
     /// affects token cost, so this lets the operator line model usage
     /// up against the cost series over time.
     pub model_counts: HashMap<String, u64>,
+    /// Turn count per `result_kind` in this bucket. Lets the stats
+    /// page chart error / rate-limit / compaction outcomes *over time*
+    /// (the window-total lives in `Snapshot::result_mix`).
+    pub result_counts: HashMap<String, u64>,
 }
 
 #[derive(Debug, Serialize)]
@@ -243,6 +247,7 @@ fn snapshot(path: &Path, window: Window) -> Result<Snapshot> {
         acc.ctx_sum = acc.ctx_sum.saturating_add(r.last_input_tokens);
         acc.ctx_max = acc.ctx_max.max(r.last_input_tokens);
         *acc.model_counts.entry(r.model.clone()).or_insert(0) += 1;
+        *acc.result_counts.entry(r.result_kind.clone()).or_insert(0) += 1;
 
         all_durations.push(r.duration_ms.max(0));
         *wake_totals.entry(r.wake_from).or_insert(0) += 1;
@@ -304,6 +309,7 @@ struct BucketAcc {
     ctx_sum: u64,
     ctx_max: u64,
     model_counts: HashMap<String, u64>,
+    result_counts: HashMap<String, u64>,
 }
 
 fn fill_buckets(
@@ -352,6 +358,7 @@ fn fill_buckets(
                 avg_ctx_tokens: avg_ctx,
                 max_ctx_tokens: acc.ctx_max,
                 model_counts: acc.model_counts.clone(),
+                result_counts: acc.result_counts.clone(),
             }
         } else {
             Bucket {
@@ -367,6 +374,7 @@ fn fill_buckets(
                 avg_ctx_tokens: 0.0,
                 max_ctx_tokens: 0,
                 model_counts: HashMap::new(),
+                result_counts: HashMap::new(),
             }
         };
         out.push(bucket);