Seed leg splits per prefix group so like accounts split targets alike

Like accounts in a batch previously carved legs with independent
Math.random, so near-identical day targets could split very differently.
Leg fractions are now drawn from groupLegFraction(prefix, tradeDate,
legIndex) — FNV-1a seeded like dailyBump — so accounts of the same
account type on the same leg of the same day split identically when
their remaining targets match, and proportionally close otherwise.

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

lib/trading-logic.ts

@@ -92,6 +92,22 @@ export function currentTradeDateCT(now: Date = new Date()): string {
     return new Date(date).toISOString().slice(0, 10);
 }
 
+/**
+ * Deterministic fraction in [0, 1) for a (prefix group, trade date, leg index) triple.
+ * Like accounts (same account-type prefix) carving the same leg of the same day draw the
+ * same fraction, so pickLegAmount splits their targets identically when their remaining
+ * amounts match and proportionally close when they differ (bumps, epsilon leftovers).
+ */
+export function groupLegFraction(prefix: string, tradeDate: string, legIndex: number): number {
+    const key = `${prefix}|${tradeDate}|${legIndex}`;
+    let hash = 0x811c9dc5;
+    for (let i = 0; i < key.length; i++) {
+        hash ^= key.charCodeAt(i);
+        hash = Math.imul(hash, 0x01000193) >>> 0;
+    }
+    return hash / 0x100000000;
+}
+
 /**
  * Pick the next leg to trade toward today's remaining target.
  * remaining ≤ BIG_DAY_THRESHOLD → trade it all in one final leg; otherwise carve a random