Report per-account-type days-traded histograms to the master

Adds daysHist ({daysTraded: accountCount}) to AccountTypeStats, computed
from the same combined-day view the trade path uses and covering every
live account including ones mid-trade. Feeds the master's payout
calendar; an older master simply ignores the field.

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

lib/master-protocol.ts

@@ -22,6 +22,10 @@ export interface AccountTypeStats {
     targetHit?: number;
     partial?: number;
     loss?: number;
+    /** Live accounts by trading days completed toward the current target:
+     *  {daysTraded: accountCount}. Feeds the master's payout calendar.
+     *  Optional so an older master simply ignores it. */
+    daysHist?: Record<number, number>;
 }
 
 export interface FirmStats {