payg bundle: FE bundlePriceMinor matches the edge-fn amount_off math

bundlePriceMinor now subtracts the rounded discount (subtotal - round(subtotal*(granted-paid)/granted)) instead of rounding the discounted price, so the pre-mint estimate matches create-payg-bundle-quote's inline amount_off coupon to the penny on exact-half ties. Same results as before except tie cases; adds a tie-case test that pins the method.

Author: ConnorYoh

frontend/editor/src/proprietary/billing/format.test.ts

@@ -54,7 +54,7 @@ describe("prepaid bundle brain", () => {
     expect(q.provisionedMonthlyVolume).toBe(10000);
     expect(q.poolCredits).toBe(576_000);
     expect(q.listMinor).toBe(576_000); // 576k credits × 1¢
-    expect(q.priceMinor).toBe(480_000); // × 10/12
+    expect(q.priceMinor).toBe(480_000); // 576,000 − round(576000×2/12)=96,000
     expect(q.savingsMinor).toBe(96_000);
     expect(q.overEnterprise).toBe(false);
   });
@@ -86,17 +86,26 @@ describe("prepaid bundle brain", () => {
   });
 
   it("applies the 12-for-10 discount at the per-run rate", () => {
-    // 120k credits × 2 minor = 240,000 list; × 10/12 = 200,000 paid.
+    // 120k credits × 2 minor = 240,000 list; minus round(240000×2/12)=40,000 = 200,000 paid.
     expect(bundleListMinor(120_000, 2)).toBe(240_000);
     expect(bundlePriceMinor(120_000, 2)).toBe(200_000);
   });
 
   it("rounds sub-cent rates to the minor unit (HALF_UP)", () => {
-    // 100k × 0.5 = 50,000 list; × 10/12 = 41,666.67 → 41,667.
+    // 100k × 0.5 = 50,000 list; minus round(50000×2/12)=round(8333.33)=8,333 = 41,667.
     expect(bundleListMinor(100_000, 0.5)).toBe(50_000);
     expect(bundlePriceMinor(100_000, 0.5)).toBe(41_667);
   });
 
+  it("rounds the discount then subtracts, matching the edge fn on exact-half ties", () => {
+    // The mechanism that kills the coupon-rounding drift: round the DISCOUNT and subtract it, not
+    // round the discounted price. On a tie the two differ by a minor unit.
+    // subtotal 3 → discount round(0.5)=1 → 2 (round(3×10/12)=round(2.5)=3 would drift).
+    expect(bundlePriceMinor(3, 1)).toBe(2);
+    // subtotal 9 → discount round(1.5)=2 → 7 (round(9×10/12)=round(7.5)=8 would drift).
+    expect(bundlePriceMinor(9, 1)).toBe(7);
+  });
+
   it("returns null money when the rate is unknown or non-positive", () => {
     expect(bundleListMinor(120_000, null)).toBeNull();
     expect(bundlePriceMinor(120_000, null)).toBeNull();

frontend/editor/src/proprietary/billing/format.ts

@@ -199,21 +199,26 @@ export function bundleListMinor(
 }
 
 /**
- * Discounted price of a prepaid pool in minor units: units × rate × paid/granted.
- * Mirror of the Stripe coupon. Null when the rate is unknown (the caller hides the
- * figure and falls back to the server total).
+ * Discounted price of a prepaid pool in minor units: the list subtotal minus the
+ * rounded 12-for-10 discount. Computed the SAME way as the edge fn that mints the
+ * Stripe coupon (create-payg-bundle-quote: round the DISCOUNT, then subtract it —
+ * not round the discounted price), so this pre-mint estimate matches the amount_off
+ * Stripe charges, and the total persisted on the quote, to the penny. The two methods
+ * diverge by a minor unit on exact-half ties. Null when the rate is unknown (the
+ * caller hides the figure and falls back to the server total).
  */
 export function bundlePriceMinor(
   units: number,
   ratePerUnitMinor: number | null | undefined,
   monthsPaid: number = PREPAID_MONTHS_PAID,
   monthsGranted: number = PREPAID_MONTHS_GRANTED,
 ): number | null {
-  const rate =
-    ratePerUnitMinor != null && ratePerUnitMinor > 0 ? ratePerUnitMinor : null;
-  return rate != null
-    ? Math.round((units * rate * monthsPaid) / monthsGranted)
-    : null;
+  const subtotal = bundleListMinor(units, ratePerUnitMinor);
+  if (subtotal == null) return null;
+  const discount = Math.round(
+    (subtotal * (monthsGranted - monthsPaid)) / monthsGranted,
+  );
+  return subtotal - discount;
 }
 
 /** Inputs to {@link computeBundleQuote} — team size + the finer-setting multipliers. */