Filenames with spaces; output beside the input file; warning fixes (from dev c08eb1bbfa4c)

- Filenames may contain spaces: quote on the command line; filename
  lists use a standalone "/" separator; unseparated names that do not
  exist are rejoined into names that do (announced); leading ~ expands.
- Without -o, output is written to the input file's directory; -o fully
  specifies the output directory and basename.
- "Word not parsed" warnings now appear without -v, only for text that
  survives removal, with correct line numbers.
- New tst/filename_test.sh regression suite.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

mac/file.h

@@ -17,6 +17,18 @@ std::string relative_pathname(const std::string& filename, const std::string& ba
 fs::path relative_to_cwd(const fs::path& input);
 
 bool file_exists(const std::string& pathname, bool error_if_not=false, bool is_directory=false);
+
+// Filenames may contain spaces.  A filename LIST in a flat string or an
+// argument vector is separated by a standalone "/" token (whitespace on both
+// sides; never a legal input filename -- "/" alone is the root directory).
+// Without a separator, whitespace-split tokens that do not name existing
+// files are greedily rejoined with their neighbors into names that do (the
+// rescue is announced).  A leading "~/" (or bare "~") expands to $HOME.
+std::string expand_tilde(const std::string& path);
+std::vector<std::string> group_filename_tokens(
+    const std::vector<std::string>& tokens, const std::string& base_dir="");
+std::vector<std::string> resolve_filename_list(
+    const std::string& text, const std::string& base_dir="");
 std::string string_from_file(const std::string& pathname, bool strip_surrounding_whitespace=false);
 void string_to_file(const std::string& pathname, std::string contents);
 std::vector<std::string> get_files_in_directory(const std::string& dir);