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/argv.h

@@ -113,5 +113,10 @@ public:
     std::vector<std::string> m_var_names {};
     std::set<std::string> m_given {};
     std::vector<std::string> m_hyphen_markers {};
+    // Original argv word boundaries for multi-word arguments (the single
+    // positional list and variadic --name options).  as_vector() returns
+    // these, so a shell-quoted filename containing spaces stays one
+    // element; the space-joined m_value remains only for get()/describe().
+    std::map<std::string, std::vector<std::string>> m_vectors {};
     long unsigned int m_syntax_size = 0;
 };