@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/link/link.k

@@ -1,15 +1,39 @@
+@@@argtype section_link |
+  treat the link target as a section title, not an id.
+
+  If ^:section is present but does not have a value, the section title is in
+  the current file.
+
+  If ^:section has a value, then it is the basename (no directory; no
+  extension) of the file in the same document that contains the section with
+  that title.
+  :pattern [A-Za-z0-9_.-]+
+  :default false
+  :alone true
+@@@
+
 @@link.k target
 :text
 :footnote.bool false 
-:section.bool false
+:section.section_link
 :nq.bool false
-:basename # xxx -- Deprecated?
 :code # xxx
 :phrase # xxx
 :dox # xxx
 :doxanchor # xxx     
 :id # xxx
-: Links within documents and to external URLs.
+:
+
+Links within documents and to external URLs.
+
+With ^:section the target is a section title, matched with klammer markup
+removed and whitespace collapsed to one space.  Written alone, ^:section
+means the section is in the current file; written with a value, the value is
+the basename of the file of the same ^@document that has the section.  A
+section link is resolved when ^@document assembles its files, so one outside
+a ^@document is an error, as is a title the file does not have, or has more
+than once (give one heading an ^:id and link to that).
+
 @@
 
 @@link.html,tex,txt :: @eval link.Link(K) eval@ @@