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/tns/md_to_sks.py

@@ -128,8 +128,17 @@ def inline(text):
     text = "".join(p if re.match(r'^[\x00\x01]', p) else quote(p) for p in parts)
 
     text = re.sub(r'\x01(\d+)\x01', lambda m: verbatim[int(m.group(1))], text)
+    # Inline code goes in RAW, and is closed with the named delimiter.  "@c"
+    # took an ordinary string parameter until 2026-08-16, so every Klammertext
+    # special inside a code span had to be quoted -- and about 40% of the
+    # inline code in the measured corpus contains one (see
+    # doc/markdown_to_klammertext.md).  It is now a LITERAL parameter, like
+    # "@code": nothing inside is interpreted, so nothing needs quoting, and the
+    # source reads as the author wrote it.  A literal klammer cannot be closed
+    # with a bare "@" -- the machine cannot find the end by counting -- so the
+    # close is "c@".
     text = re.sub(r'\x00(\d+)\x00',
-                  lambda m: OPEN + "@c " + quote(spans[int(m.group(1))]) + " @" + CLOSE, text)
+                  lambda m: OPEN + "@c " + spans[int(m.group(1))] + " c@" + CLOSE, text)
     # Klammertext delimiters need whitespace around them, but Markdown
     # emphasis abuts its neighbours: "un**bold**ed" and "*name*_rectified"
     # both put a word character hard against a delimiter, which the