test(service): pin the platform default, not just the explicit flag

Every existing case passed canRespawn explicitly, so the derivation itself
was untested: a regression to `?? false` would have left all five green
while Windows silently lost the restart window that lidge-jun#764 needs.

The new case drives both platforms through the default with a mocked
process.platform -- Linux completes at fake-clock 0 after one probe, Windows
polls past 7000. Ablation to `?? false`: 5 pass / 1 fail; restored 6/6.

Worth recording why the default is right, since the review checked it
properly: the launchd plist does set KeepAlive and the systemd unit uses
Restart=on-failure, so both CAN restart a child. But `ocx service stop`
unloads the launchd job and issues a blocking `systemctl --user stop`, and
neither is the crash exit those policies react to. The Windows scheduler
wrapper surviving `schtasks /end` remains the one real survivor case.

Author: lidge-jun

tests/service-stop-verification.test.ts

@@ -93,4 +93,36 @@ describe("service stop verification (#764)", () => {
     });
     expect(live).toEqual({ port: 10100 });
   });
+
+  test("the platform default drives the behavior, not just the explicit flag", async () => {
+    // Every other test passes canRespawn explicitly, so none of them exercises the DEFAULT --
+    // a regression to `?? false` would leave them all green while Windows silently lost the
+    // restart window it needs. These pin the derivation itself.
+    const original = process.platform;
+    const setPlatform = (value: string) =>
+      Object.defineProperty(process, "platform", { value, configurable: true });
+    try {
+      setPlatform("linux");
+      const linuxClock = fakeClock();
+      let linuxProbes = 0;
+      await proxyStillLiveAfterStop({
+        findProxy: async () => { linuxProbes += 1; return null; },
+        ...linuxClock,
+      });
+      expect(linuxProbes).toBe(1);
+      expect(linuxClock.now()).toBe(0);
+
+      setPlatform("win32");
+      const winClock = fakeClock();
+      let winProbes = 0;
+      await proxyStillLiveAfterStop({
+        findProxy: async () => { winProbes += 1; return null; },
+        ...winClock,
+      });
+      expect(winProbes).toBeGreaterThan(1);
+      expect(winClock.now()).toBeGreaterThanOrEqual(7000);
+    } finally {
+      Object.defineProperty(process, "platform", { value: original, configurable: true });
+    }
+  });
 });