feat: report authoritative playback progress from shared backend

Sources/CLI/TerebiCLI.swift

@@ -17,7 +17,16 @@ enum TerebiCLI {
             print(PlayerCommand.usage)
         case .printDefaultConfig:
             print(PlayerConfiguration.defaultTOML, terminator: "")
-        case .launch(let url, let newInstance, let noConfig):
+        case .launch(let url, let newInstance, let noConfig, let progressFile):
+            if let progressFile {
+                var existing = stat()
+                guard lstat(progressFile.path, &existing) != 0 else {
+                    fail("Progress file already exists: \(progressFile.path). Supply a nonexistent destination.", code: 73)
+                }
+                guard errno == ENOENT else {
+                    fail("Could not inspect progress file \(progressFile.path): \(String(cString: strerror(errno))).", code: 73)
+                }
+            }
             let configURL = PlayerConfiguration.fileURL(environment: ProcessInfo.processInfo.environment,
                 homeDirectory: FileManager.default.homeDirectoryForCurrentUser)
             if !noConfig {
@@ -39,6 +48,9 @@ enum TerebiCLI {
             configuration.createsNewApplicationInstance = newInstance
             // LaunchServices does not inherit the invoking shell's XDG environment.
             configuration.arguments = noConfig ? ["--no-config"] : ["--config-file", configURL.absoluteString]
+            if let progressFile {
+                configuration.arguments += ["--progress-file", progressFile.path]
+            }
             if newInstance, let url {
                 configuration.arguments += ["--open-media", url.absoluteString]
             }