@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/section/section.py

@@ -49,10 +49,15 @@ class Section(klammer_base.Klammer_base):
         # names = "part chapter section subsection subsubsection paragraph subparagraph subsubparagraph subsubsubparagraph".split()
         names = "part sA sB sC sD sE".split()
 
+        # The \hypertarget name.  K_document_file is the chapter's basename,
+        # set by @document (document_latex.cpp): with it the name is stable
+        # across the page cache, where a counter alone collided when some
+        # chapters were served from cache and others reprocessed.
+        basename = getattr(self, "K_document_file", "") or ""
         if self.id:
             id = self.id
         else:
-            id = f"-s{Section.label_number}"
+            id = f"{basename}-s{Section.label_number}"
             Section.label_number += 1
 
         result = ''
@@ -64,7 +69,10 @@ class Section(klammer_base.Klammer_base):
 
         # result = kutil.protect_klammertext_special_characters(result)
 
-        result = f"%__sectionlink__{id}__{self.title}__" + "\n" + result
+        # The announcement @document's section-link resolver reads (fields
+        # separated by KTSL; see sks/document/section_link.h).  Title last:
+        # it may hold anything.
+        result = f"%__sectionlink__{id}__KTSL__{basename}__KTSL__{self.title}__" + "\n" + result
         return result
 
     def txt(self):