feat: print complete default configuration from CLI

Checks/configuration.swift

@@ -110,6 +110,10 @@ struct ConfigurationChecks {
         let fallback = home.appendingPathComponent(".config/terebi/config.toml")
         let config = xdg.appendingPathComponent("terebi/config.toml")
         let defaults = PlayerConfiguration()
+        let printed = root.appendingPathComponent("printed-defaults.toml")
+        try Data(PlayerConfiguration.defaultTOML.utf8).write(to: printed)
+        try require(try PlayerConfiguration.load(from: printed) == defaults,
+                    "printed TOML is directly usable and preserves all default behavior and bindings")
         try require(!defaults.showBufferInfo && !defaults.automaticQuality && defaults.snapToEdges,
                     "built-in defaults hide buffer info, disable automatic quality, and enable snapping")
         try require(PlayerConfiguration.fileURL(environment: ["XDG_CONFIG_HOME": xdg.path], homeDirectory: home) == config,

Checks/player-command.swift

@@ -55,6 +55,9 @@ struct PlayerCommandChecks {
         guard case .help = try PlayerCommand.parse(["--new-instance", "--help"]) else {
             fatalError("FAIL: help must not launch an independent player")
         }
+        guard case .printDefaultConfig = try PlayerCommand.parse(["--print-default-config"]) else {
+            fatalError("FAIL: printing defaults must not launch a player")
+        }
 
         for arguments in [["--url"], ["--url", canonical.absoluteString, movie.path], ["--"],
                           [movie.path, movie.path], ["--unknown"], ["missing.mp4"], [directory.path],
@@ -62,6 +65,7 @@ struct PlayerCommandChecks {
                           ["--new-instance", "--new-instance"], ["--new-instance", "--url"],
                           ["--new-instance", "--"], ["--new-instance", movie.path, movie.path],
                           ["--new-instance", "--unknown"], ["--no-config", "--no-config"],
+                          ["--print-default-config", movie.path], ["--print-default-config", "--print-default-config"],
                           ["--no-config", "--new-instance", "--no-config"]] {
             do {
                 _ = try PlayerCommand.parse(arguments, directory: directory)