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>

com/kdesc.cpp

@@ -107,7 +107,7 @@ int main(int argc, char* argv[])
         // (infrastructure) and load no klammer set.
         if (args.given("font")) {
             strings_t words {};
-            for (const std::string& w : word_split(args.get("font"))) {
+            for (const std::string& w : args.as_vector("font")) {
                 if (!w.empty()) {
                     words.push_back(w);
                 }
@@ -118,7 +118,7 @@ int main(int argc, char* argv[])
 
         Machine M;
 
-        strings_t input_filenames = args.as_vector("input");
+        strings_t input_filenames = resolve_filename_list(args.get("input"));
         std::cout << "input_filenames: " << input_filenames << "\n";
         if (input_filenames.empty()) {
             M.read(fs::path(M.m_state.value("KLAMMERTEXT_HOME") + "/sks/sks.k"));