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>

sks/image/image.py

@@ -31,7 +31,7 @@ class Image(klammer_base.Klammer_base):
         self.basename = klammer_base.unescape_ktesc(self.basename)
         self.source, self.pwidth, self.pheight, self.file_error  = self.cache.get(self.K_target, self.basename)
         if self.file_error:
-            self.file_error_message = f'\nERROR: File "{self.K_input_filenames}" not found'
+            self.file_error_message = f'\nERROR: File "{self.K_input_filename}" not found'
         self.as_string = as_string
         if width:
             self.width = width
@@ -42,7 +42,7 @@ class Image(klammer_base.Klammer_base):
             self.rel_fraction = self.pwidth / self.rel_pwidth
 
         if self.file_error:
-            self.file_error_message = f'\nERROR: File "{self.K_input_filenames}" not found'
+            self.file_error_message = f'\nERROR: File "{self.K_input_filename}" not found'
 
     def html(self):
         img_dir = f"{self.K_output_dir}/{self.K_output_basename}/{self.Image_output_dir}"