Literal @c, @source_listing with :marker, and a large-directory speedup

Three changes.

@c now takes its content literally, like @code -- it is the inline form
and @code the block form of the same thing.  The named close "c@" is
required, and characters that are special in a target no longer break
the file: @c a_b c@ renders correctly everywhere.  The Markdown
converter stops quoting inline code, since nothing needs protecting.

@source_file is renamed @source_listing.  Code read from a file is its
own klammer; @code is only for a block written inline (its never-
implemented :filename and :pattern options are removed).  The new
:marker P option lists the region between two lines that are exactly
//P, so the source file declares its own extractable regions.  A marker
missing or not appearing exactly twice is an error, never a fallback.

Rendering a document that sits in a large directory was paying a
recursive walk of that directory's whole tree on every @eval -- 27
seconds for a document that renders in a third of one.  The walk is now
a non-recursive look decided once per directory.

Assembled from dev commit 071b1b183de4.

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

sks/code/code.k

@@ -1,15 +1,27 @@
 
-@@code.k :filename :pattern
+@@code.k
   @hpos_args :hpos left @
   @caption_args :caption_side top @
   | text.literal :
-A source file displayed verbatim
+A block of code, given here and displayed uninterpreted.  Code that lives in a
+file is ^@source_listing instead; ^@code is only for a block written inline.
+
+It carried ":filename" and ":pattern" until 2026-08-16.  Neither was ever
+implemented -- "filename" is read only by ^@source_listing and "pattern" by
+nothing at all -- and because a literal parameter's content begins at the first
+katom when no bar is written, "^@code :filename f :pattern p" DISPLAYED THOSE
+WORDS as the listing rather than acting on them.
 @@
 
 @@code.html,tex :: @eval code_block.Code(K) @ @@
 
-@@c.k code_text :
-A word or phrase displayed verbatim in a line
+@@c.k code_text.literal :
+A word or phrase displayed verbatim in a line.  Like ^@code, its content is
+LITERAL -- nothing in it is interpreted as Klammertext -- so it must be closed
+with the named delimiter "c^@".  ^@c is the in-line form and ^@code the block
+form of the same thing; before 2026-08-16 this parameter was an ordinary
+string, so a "^#" or a bare "^@" inside it was read as Klammertext and usually
+failed the file, which is not what "verbatim" can mean.
 @@
 
 @@c.html,tex :: @eval code_block.Code_fragment(K) eval@
@@ -17,5 +29,19 @@ A word or phrase displayed verbatim in a line
 
 # :cwd makes the filename resolve against the DOCUMENT's directory, not
 # the directory ktext happens to run in.
-@@source_file.k filename : Display the contents of the file verbatim. @@
-@@source_file.html,tex :: @eval :cwd *K_input_dir* code_block.Source(K) @ @@
+@@source_listing.k filename :marker
+  @hpos_args :hpos left @
+  @caption_args :caption_side top @
+  : Code read from a file and displayed uninterpreted.  *filename* is the file;
+without ":marker" the whole file is listed.
+
+":marker P" lists only the region BETWEEN two lines that consist solely of
+"//P" and begin in the first column.  The source file therefore declares its
+own extractable regions and the document asks for one by name, so the two
+cannot drift apart silently: renaming or reformatting the code does not change
+what is extracted, and a region that disappears is an error rather than a
+quietly different listing.
+
+Named "^@source_file" until 2026-08-16.  # retired-ok
+@@
+@@source_listing.html,tex :: @eval :cwd *K_input_dir* code_block.Source(K) @ @@