feat: support unassigned purchases and allocation status

Allow month-only purchases to start finance plans. Keep unbudgeted posts distinct from explicit zero rules in overspend reporting.

src/domain/types.ts

@@ -169,7 +169,7 @@ export interface RecurringCost {
   endedFrom?: IsoDate;
   recurrence: Recurrence;
   anchoring: Anchoring;
-  /** Same shape and same invariant as Purchase: exactly one absorbsRemainder. */
+  /** Bills require exactly one remainder-absorbing destination. */
   splitMode: "percent" | "fixed";
   splits: Split[];
 }
@@ -183,6 +183,7 @@ export interface Purchase {
   /** In the purchase's own currency. */
   total: Money;
   splitMode: "percent" | "fixed";
+  /** Empty means spent in the month without charging any post. */
   splits: Split[];
   /** null means the whole total is charged in the month of `date`. */
   schedule: Schedule | null;