.ssp: add .children property for heading tree navigation

- Add explicit child heading OCN lists to heading objects,
  pre-computed in a single O(n) pass over the body section
  before serialization. This makes the document tree directly
  navigable without scanning - each heading lists its direct
  sub-heading OCNs.

- Example output for a chapter heading:
    [10] heading :1
    .last_descendant: 65
    .children: 14 24 42 57

- Implementation: builds an int[][int] map (parent_ocn ->
  child heading OCNs) from one pass over the body objects,
  then emits .children: during serialization for headings
  that have entries in the map.

- The tree was already reconstructable from parent_ocn +
  last_descendant_ocn, but .children makes it immediate -
  no scanning required to find a heading's sub-structure.

- Tested against all 35 sample documents - zero failures.

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

1 files changed, 19 insertions, 0 deletions

diff --git a/src/sisudoc/io_out/create_abstraction_txt.d b/src/sisudoc/io_out/create_abstraction_txt.d
index 7d0425c..d393f61 100644
--- a/src/sisudoc/io_out/create_abstraction_txt.d
+++ b/src/sisudoc/io_out/create_abstraction_txt.d
@@ -162,6 +162,16 @@ 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"];
@@ -201,6 +211,15 @@ template spineAbstractionTxt() {
         if (obj.metainfo.last_descendant_ocn != 0)
           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) {
+          string[] ch;
+          foreach (c; heading_children[obj.metainfo.ocn]) {
+            ch ~= c.to!string;
+          }
+          output ~= ".children: " ~ ch.join(" ");
+        }
+
         /+ ↓ ancestors (only if non-zero) +/
         {
           bool has_anc = false;