scm: support --eval

With the ~--eval~ option you can evaluate an expression in mu/scm
environment. For example:

$ mu scm --eval \
  '(format #t "found ~d match(es)\n" (length (mfind "hello")))'
found 7173 match(es)

Add command-line parameter and implement.

mu/mu-cmd.cc

@@ -1,5 +1,5 @@
 /*
-** Copyright (C) 2010-2025 Dirk-Jan C. Binnema <djcb@djcbsoftware.nl>
+** Copyright (C) 2010-2026 Dirk-Jan C. Binnema <djcb@djcbsoftware.nl>
 **
 ** This program is free software; you can redistribute it and/or modify it
 ** under the terms of the GNU General Public License as published by the
@@ -75,7 +75,9 @@ cmd_scm(const Store& store, const Options& opts)
 #else
 	if (opts.scm.script_path)
 		return Mu::Scm::run_script(store, opts, *opts.scm.script_path);
-	else
+	else if (opts.scm.eval)
+		return Mu::Scm::run_eval(store, opts, *opts.scm.eval);
+		else
 		return Mu::Scm::run_repl(store, opts, opts.scm.socket_path.value_or(""));
 #endif /*BUILD_SCM*/
 }

mu/mu-options.cc

@@ -650,7 +650,9 @@ sub_scm(CLI::App& sub, Options& opts)
 		->excludes("--listen");
 	sub.add_option("script-args", opts.scm.params, "Parameters for script")
 		->type_name("<parameters>");
-
+	sub.add_option("--eval", opts.scm.eval, "Expression to evaluate")
+		->excludes("--listen")
+		->excludes("script-path");
 }
 
 static void

mu/mu-options.hh

@@ -1,5 +1,5 @@
 /*
-** Copyright (C) 2022-2025 Dirk-Jan C. Binnema <djcb@djcbsoftware.nl>
+** Copyright (C) 2022-2026 Dirk-Jan C. Binnema <djcb@djcbsoftware.nl>
 **
 ** This program is free software; you can redistribute it and/or modify it
 ** under the terms of the GNU General Public License as published by the
@@ -261,6 +261,7 @@ struct Options {
 		bool listen; /**< Whether to start listening on a socket */
 		Option<std::string> socket_path; /**< path for the '--listen'
 						  * Unix domain socket */
+		Option<std::string> eval; /**< expression to evaluate */
 	} scm;