split the integration harness into server, api and util modules

tools/integration-test.nu is now just the entrypoint. The server lifecycle and fixture preparation live in tools/integration/servers.nu, the Subsonic request helpers in subsonic.nu, and shared waiting/port helpers in util.nu. No behaviour change; the assertions still live in the Kotlin tests.

tools/integration/util.nu

@@ -0,0 +1,15 @@
+# Shared helpers for the integration harness: waiting and port checks.
+
+export def poll [label: string, check: closure, --attempts (-n): int = 120] {
+    for attempt in 0..<$attempts {
+        let value = (try { do $check } catch { null })
+        if $value != null { return $value }
+        if $attempt > 0 and ($attempt mod 5) == 0 { print $"waiting for ($label)..." }
+        sleep 1sec
+    }
+    error make { msg: $"timed out waiting for ($label)" }
+}
+
+export def port-in-use [port: int] {
+    (^curl -s -o /dev/null --max-time 1 $"http://127.0.0.1:($port)/" | complete).exit_code != 7
+}