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>

mac/state.cpp

@@ -1,5 +1,6 @@
 #include <regex>
 #include <iostream>
+#include <map>
 
 #include "show.h"
 #include "file.h"
@@ -10,6 +11,35 @@
 
 int State::class_id = 0;
 
+namespace {
+
+// A sys.path entry reaches only the directory's own files, so the decision
+// must not recurse; and the answer is fixed for the run (the search-dir set
+// only grows, and a directory's answer does not change), so it is decided
+// once per directory per process. The former recursive walk here ran on
+// every @eval and took ~200 ms per eval on a document in a large home
+// directory (27 s for one render).
+bool directory_has_python_file(const std::string& dir)
+{
+    static std::map<std::string, bool> cache {};
+    auto it = cache.find(dir);
+    if (it != cache.end()) {
+        return it->second;
+    }
+    bool found = false;
+    std::error_code ec {};
+    for (const auto& entry : std::filesystem::directory_iterator(dir, ec)) {
+        if (entry.is_regular_file(ec) && entry.path().extension() == ".py") {
+            found = true;
+            break;
+        }
+    }
+    cache[dir] = found;
+    return found;
+}
+
+} // namespace
+
 bool Var::defined() const
 {
     return !m_name.empty();
@@ -297,8 +327,7 @@ std::string State::python_code()
     python_dirs.insert(python_dirs.end(),
                        m_search_dirs.begin(), m_search_dirs.end());
     for (const auto& d : python_dirs) {
-        auto python_files = pathnames_with_extension(d, "py");
-        if (!python_files.empty()) {
+        if (directory_has_python_file(d)) {
             ss << "sys.path.append('" << d << "')\n";
         }
     }