fix: only count matured setups in the live track record

The outcome stats were dominated by quick stop-outs: near stops resolve as losses
within days while far targets take weeks, so a young sample (mostly pending,
0 expired) skewed sharply negative (e.g. 13.8% hit / -0.46R vs the backtest's
35.8% / +0.18R) — a maturation artifact, not a real result.

get_performance_stats now counts only setups whose full ~30-day window has
elapsed (_MATURITY_DAYS), so winners had as long as losers (unbiased, and
comparable to the backtest). A new `maturing` count reports the younger setups
held back. The Track Record UI relabels "Evaluated" -> "Matured", shows the
maturing count, and explains the window in the empty state + methodology note.

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

frontend/src/lib/types.ts

@@ -153,6 +153,7 @@ export interface OutcomeBucketStats {
 export interface PerformanceStats {
   overall: OutcomeBucketStats;
   pending: number;
+  maturing: number;
   by_direction: Record<string, OutcomeBucketStats>;
   by_action: Record<string, OutcomeBucketStats>;
   by_confidence: Record<string, OutcomeBucketStats>;