@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/phases.py

@@ -417,3 +417,20 @@ def html_format_footnotes(text, K=None):
         # Temporarily add room for footnotes to move to the top of the window for the test:
         result += '<div style="height: 100lh"></div>\n'
     return result
+
+
+
+# @link :section outside a @document (2026-10-12).  A section link is
+# resolved when @document assembles its chapters, so one that is applied
+# outside any document can never be resolved: its marker would reach the
+# output.  @document sets K_document_file on each chapter's Machine; a
+# Machine without it that still holds the marker after its own apply is
+# that case.  (Inside a document the outer Machine's apply ends with the
+# resolved text -- html writes files and returns nothing -- so the marker
+# is gone by the time this runs there.)
+def check_section_links(text, K):
+    if "__seclink__" in text and not getattr(K, "K_document_file", ""):
+        raise ValueError("A @link with :section names a section by its heading, "
+                         "which only a @document can resolve, and this link is "
+                         "outside any @document")
+    return text