add children_headings to document abstraction

Add int[] children_headings field to DocObj_MetaInfo_ and
compute it in the post-processing pass of metadoc_from_src.d,
right after last_descendant_ocn. Single O(n) pass builds a
parent_ocn -> child heading OCNs map, then assigns to each
heading object. Useful for tree-structured output.

The .ssp serializer now reads directly from the abstraction
field instead of pre-computing its own map.

  metadoc_object_setter.d: +1 line (field declaration)
  metadoc_from_src.d: +17 lines (computation)
  create_abstraction_txt.d: -10 lines (simplified)

Co-Authored-By: Anthropic Claude Opus 4.6 (1M context)

1 files changed, 2 insertions, 12 deletions

diff --git a/src/sisudoc/io_out/create_abstraction_txt.d b/src/sisudoc/io_out/create_abstraction_txt.d
index f92b83c..af98f61 100644
--- a/src/sisudoc/io_out/create_abstraction_txt.d
+++ b/src/sisudoc/io_out/create_abstraction_txt.d
@@ -162,16 +162,6 @@ template spineAbstractionTxt() {
     output ~= "}";
     output ~= "";
 
-    /+ ↓ pre-compute heading children map (one pass over body) +/
-    int[][int] heading_children;  // parent_ocn -> [child heading OCNs]
-    if ("body" in doc_abstraction) {
-      foreach (obj; doc_abstraction["body"]) {
-        if (obj.metainfo.is_a == "heading" && obj.metainfo.parent_ocn != 0) {
-          heading_children[obj.metainfo.parent_ocn] ~= obj.metainfo.ocn;
-        }
-      }
-    }
-
     /+ ↓ document sections +/
     string[] section_order = ["head", "toc", "body", "endnotes",
       "glossary", "bibliography", "bookindex", "blurb"];
@@ -212,9 +202,9 @@ template spineAbstractionTxt() {
           output ~= ".last_descendant: " ~ obj.metainfo.last_descendant_ocn.to!string;
 
         /+ ↓ child headings (from pre-computed map) +/
-        if (obj.metainfo.is_a == "heading" && obj.metainfo.ocn in heading_children) {
+        if (obj.metainfo.children_headings.length > 0) {
           string[] ch;
-          foreach (c; heading_children[obj.metainfo.ocn]) {
+          foreach (c; obj.metainfo.children_headings) {
             ch ~= c.to!string;
           }
           output ~= ".children: " ~ ch.join(" ");