test(player): pin the stop URL's ids, the beacon's settle, the kept point's edges

The stop-URL contract passed with the two ids swapped and read the backend
from the working directory; nothing called a real beacon's onSettled or its
library pass-through; the kept-point rule was held by its constants only.

frontend/src/lib/resume.dom.test.ts

@@ -154,6 +154,18 @@ describe("keptPosition (the server's rule)", () => {
     expect(keptPosition(2995, 3000)).toBe(0);
     expect(keptPosition(120, 0)).toBe(120); // an unknown length cannot be "near the end"
   });
+
+  // The edges themselves, one second either side: a `>=` that became `>` on one side only moves a
+  // rule both sides must apply alike, and the constants alone (resume.contract.test.ts) would not see
+  // it. backend/tests/test_library_progress.py holds the server to the same rows.
+  it.each([
+    [4, 3000, 0],
+    [5, 3000, 5],
+    [2990, 3000, 2990],
+    [2991, 3000, 0],
+  ])("at %s s of %s keeps %s", (position, duration, kept) => {
+    expect(keptPosition(position, duration)).toBe(kept);
+  });
 });
 
 describe("a shared link's point", () => {