fix: prevent idle sleep during playback

Checks/playback-power.swift

@@ -0,0 +1,19 @@
+import Foundation
+import IOKit.pwr_mgt
+
+// Inspect this process, not global assertions from audio drivers or other apps.
+func requirePlaybackPower(system: Bool, display: Bool, _ context: String) {
+    var result: Unmanaged<CFDictionary>?
+    precondition(IOPMCopyAssertionsByProcess(&result) == kIOReturnSuccess,
+                 "Could not inspect macOS power assertions")
+    let byProcess = result!.takeRetainedValue() as NSDictionary
+    let assertions = byProcess[NSNumber(value: ProcessInfo.processInfo.processIdentifier)] as? [[String: Any]] ?? []
+    let active = assertions.filter { ($0[kIOPMAssertionLevelKey] as? NSNumber)?.intValue != 0 }
+    let types = Set(active.compactMap { $0[kIOPMAssertionTypeKey] as? String })
+    guard types.contains(kIOPMAssertPreventUserIdleSystemSleep) == system &&
+          types.contains(kIOPMAssertPreventUserIdleDisplaySleep) == display else {
+        FileHandle.standardError.write(Data(
+            "FAIL: \(context): expected system=\(system), display=\(display); actual=\(types)\n".utf8))
+        exit(EXIT_FAILURE)
+    }
+}