fix: judge robustness under the recommended exit, not the abandoned one

The robustness warning was computed on the target-model distribution
while the same panel recommends the hold exit — internally inconsistent.
_robustness_stats (median, profit factor, ex-top-5% expectancy) is now
shared by _bucket_stats and _time_exit_bucket, the time-exit table shows
Median Net R and Ex-Top-5% per hold length, and _build_recommendation
reads the trimmed expectancy from the recommended exit's bucket (falling
back to the target model when no hold is recommended).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/lib/types.ts

@@ -259,6 +259,9 @@ export interface BacktestTimeExitRow {
   worst_r?: number | null;
   avg_hold_days?: number | null;
   net_r_per_day?: number | null;
+  median_net_r?: number | null;
+  profit_factor?: number | null;
+  net_avg_r_ex_top5?: number | null;
 }
 
 export interface BacktestPortfolioPolicy {