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/kutil/kutil.k

@@ -69,3 +69,14 @@ An <id> is the value of the ^:id argument for an image.
 
 ^:pattern before(?^:\s+\d+)?^|after(?^:\s+\d+)?^|[-\w]+
 @@@
+
+@@@argtype filename_list |
+  one or more filenames.  Filenames may contain spaces: a list is separated
+  by a standalone "/" (whitespace on both sides), e.g.
+  "chapter 1.kt / chapter 2.kt".  Without the separator, the names are
+  separated by whitespace, and names that do not exist are rejoined with
+  their neighbors into names that do.  A leading ~ expands to the home
+  directory.
+  :pattern [\s\S]*
+  :python_cast (lambda s: __import__("kutil").filename_list(s))
+@@@