@link :section links to a section by its heading, with an alone value (from dev 831455f36fcd)

A section link names the TEXT of a heading: :section alone means the current
file, a value names the file of the same @document that has it. Resolved by
@document at assembly in html and tex; errors for a missing file, a missing
title, a doubled title and a link outside a @document. The Markdown
converter reads a Markdown anchor as a section link to the heading its slug
names. Also from dev: the converter's four silent defects fixed and --chapter
(9ae2c4cd), and the engine's alone-value error text.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

mac/argtype_registry.cpp

@@ -113,12 +113,12 @@ void Argtype_registry::add(const std::string& name, const std::string& desc,
         // which is equally unable to delimit.
         if (std::regex_match(std::string("one two"), m_types[name].m_regex)) {
             std::stringstream ss {};
-            ss << "The argument type \"" << name << "\" cannot declare an :alone value"
+            ss << "The argument type \"" << name << "\" cannot declare an :alone value "
                << "because its pattern matches running text:"
                << "  " << pattern << "\n\n"
-               << "An :alone value is used when an option name is written without a" 
-               << "value.  A type that matches running text cannot tell a bare option"
-               << "name from one whose value follows it, so the text after the name"
+               << "An :alone value is used when an option name is written without a "
+               << "value.  A type that matches running text cannot tell a bare option "
+               << "name from one whose value follows it, so the text after the name "
                << "would be taken as the value instead.";
             throw Definition_error(ss.str(), loc);
         }