@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>

sks/target/target.k

@@ -102,7 +102,8 @@ of this: neither the transforms nor the quoting reach it.
   ' ’         |
   ~ &^#160;
   :after_apply
-    phases.html_format_footnotes
+    phases.html_format_footnotes ;
+    phases.check_section_links
 @@@
 
 # No transforms: LaTeX applies the input conventions itself.  The :resolve
@@ -116,7 +117,7 @@ of this: neither the transforms nor the quoting reach it.
 @@@target tex | LaTeX
   :escape \ \textbackslash{} & \& { \{ } \} $ \$ % \% _ \_ ^# \^# ^^ \textasciicircum{}
   :resolve - {-} ~ \textasciitilde{}
-  :after_apply phases.tex_resolve_markers
+  :after_apply phases.tex_resolve_markers ; phases.check_section_links
 @@@
 
 @@@target pdf | PDF from LaTeX