test(player): hold the session stop and the kept-point rule to the server

- The client's resume thresholds equal the server's (read from both sources).
- The session stop's URL matches the route the server serves it on.
- The player stops its server session by its item and its own id on an unload and on close,
  and keeps it when the page only goes into the back/forward cache.

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

@@ -0,0 +1,31 @@
+import { readFileSync } from "node:fs";
+import { expect, it } from "vitest";
+
+// One rule, two languages, no shared wire format — so a test is the only thing that holds them
+// together. Which point a library item keeps is the server's (`routes/_common.kept_position`); the
+// client repeats it (`lib/resume.ts`) for the shared link, which has no server side, and for the
+// library row it patches before the server has answered. A server-only change left the row showing
+// a Continue point the server had already cleared, and a link resuming at points the library no
+// longer keeps. Both must be EQUAL: neither direction is merely conservative here.
+//
+// Read from source rather than imported, as in plexFinishMargin.contract.test.ts.
+function constantIn(path: string, pattern: RegExp): number {
+  const source = readFileSync(new URL(path, import.meta.url), "utf8");
+  const m = pattern.exec(source);
+  // If this fails the constant was renamed or moved — find it and re-point this test, because the
+  // rule it guards did not go away.
+  expect(m, `${pattern} not found in ${path}`).not.toBeNull();
+  return Number(m![1]);
+}
+
+const SERVER = "../../../backend/app/routes/_common.py";
+
+it.each([
+  ["the least position that counts as started", "PROGRESS_MIN_SECONDS", "PROGRESS_MIN_S"],
+  ["the margin before the end that counts as finished", "FINISH_MARGIN_SECONDS", "FINISH_MARGIN_S"],
+])("keeps %s the same as the server", (_what, server, client) => {
+  const backend = constantIn(SERVER, new RegExp(`^${server}\\s*=\\s*(\\d+)`, "m"));
+  const frontend = constantIn("./resume.ts", new RegExp(`^const ${client}\\s*=\\s*(\\d+)`, "m"));
+  expect(backend).toBeGreaterThan(0);
+  expect(frontend).toBe(backend);
+});