test: correct the seed priority comment of the onboarding samples test (#384)

The seed is not all priority 0: COSTCO and WALMART are at 50, COUCHE-TARD
ESSENCE at 10. The comment now says why the mock's sort stays right: it
sorts by descending priority like the SQL, and no line of either sample is
matched by more than one seed keyword, so the order cannot move the counts.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/__integration__/onboarding-samples.test.ts

@@ -225,8 +225,13 @@ describe("onboarding samples — seed categorisation (#354)", () => {
     "%s: the seed categorises the frozen share and leaves the hand-classified lines alone",
     async (name) => {
       const keywords = seedKeywords();
-      // `categorizeBatch` asks for active keywords by descending priority; the
-      // seed is all priority 0, so the SQL order is the file order.
+      // `categorizeBatch` asks for the active keywords by descending priority
+      // and keeps the first one that matches. The seed is not all priority 0:
+      // COSTCO and WALMART are at 50, COUCHE-TARD ESSENCE at 10, the rest at 0.
+      // The mock sorts the same way; its sort is stable, so equal priorities
+      // keep the file order, which SQLite does not promise. Neither order moves
+      // the counts below: on 2026-10-18 no line of either sample matched more
+      // than one seed keyword (COSTCO WHOLESALE matches COSTCO alone).
       select.mockResolvedValue(
         [...keywords].sort((a, b) => b.priority - a.priority)
       );