Add a Vitest unit test suite (cart totals, invoice tax grouping, bundle contents)

No test infrastructure existed in this repo yet. Covers the pure logic
most likely to silently produce wrong numbers on a live order: discount/
shipping math, per-rate invoice grouping, and bundle-contents string
building. Extracted describeBundleContents() out of the checkout route
into its own module so it's importable from a test (route.ts files only
allow HTTP-method exports).

app/lib/bundleContents.ts

@@ -0,0 +1,21 @@
+import type { RawProduct } from "./productsServer";
+
+// A bundle's `bundleItems` is only ever resolved once, at checkout, into a
+// plain readable string — the order line stores this snapshot
+// (`items[].bundleContents`), not a structured sub-list, so a later change
+// to the bundle's own composition can never rewrite what a past order
+// actually contained. `product.bundleItems[].product` is a relationship
+// resolved by fetchProductsBySlug's depth:2 fetch — {id, name} objects
+// when populated, a bare id if depth somehow didn't reach it (skipped).
+//
+// Extracted out of app/api/checkout/route.ts into its own module (not
+// just kept as a local function there) so it's importable from a unit
+// test — Next.js route.ts files only allow HTTP-method + a few config
+// exports, not arbitrary named exports.
+export function describeBundleContents(product: RawProduct): string | null {
+  if (!product.bundleItems || product.bundleItems.length === 0) return null;
+  return product.bundleItems
+    .map((line) => (typeof line.product === "object" ? `${line.quantity}× ${line.product.name}` : null))
+    .filter((s): s is string => Boolean(s))
+    .join(", ");
+}