.ssp serializer: omit identifier when it equals OCN

- For heading objects, the identifier was always emitted on the
  declaration line (e.g. "[10] heading :1 10") even when it was
  just the OCN repeated. Now only emits the identifier when it
  differs from the OCN (i.e. when there is a named segment like
  "acknowledgments" or "a1"), reducing redundancy.

  Before: [10] heading :1 10
  After:  [10] heading :1

  Named segments still appear: [0] heading :1 a1

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

1 files changed, 6 insertions, 3 deletions

diff --git a/src/sisudoc/io_out/create_abstraction_txt.d b/src/sisudoc/io_out/create_abstraction_txt.d
index a1e8f12..140a27d 100644
--- a/src/sisudoc/io_out/create_abstraction_txt.d
+++ b/src/sisudoc/io_out/create_abstraction_txt.d
@@ -180,7 +180,8 @@ template spineAbstractionTxt() {
         if (obj.metainfo.is_a == "heading") {
           string lev = obj.metainfo.marked_up_level;
           obj_decl ~= "heading :" ~ lev;
-          if (obj.metainfo.identifier.length > 0) {
+          if (obj.metainfo.identifier.length > 0
+            && obj.metainfo.identifier != obj.metainfo.ocn.to!string) {
             obj_decl ~= " " ~ obj.metainfo.identifier;
           }
         } else {
@@ -191,7 +192,8 @@ template spineAbstractionTxt() {
         /+ ↓ properties (only non-default values) +/
         if (obj.metainfo.is_of_part.length > 0)
           output ~= ".part: " ~ obj.metainfo.is_of_part;
-        if (obj.metainfo.is_of_section.length > 0 && obj.metainfo.is_of_section != section)
+        if (obj.metainfo.is_of_section.length > 0
+          && obj.metainfo.is_of_section != section)
           output ~= ".section: " ~ obj.metainfo.is_of_section;
         if (obj.metainfo.parent_ocn != 0)
           output ~= ".parent: " ~ obj.metainfo.parent_ocn.to!string;
@@ -260,7 +262,8 @@ template spineAbstractionTxt() {
         /+ ↓ tag properties +/
         if (obj.tags.in_segment_html.length > 0)
           output ~= ".segment: " ~ obj.tags.in_segment_html;
-        if (obj.tags.anchor_tag_html.length > 0 && obj.tags.anchor_tag_html != obj.tags.in_segment_html)
+        if (obj.tags.anchor_tag_html.length > 0
+          && obj.tags.anchor_tag_html != obj.tags.in_segment_html)
           output ~= ".anchor: " ~ obj.tags.anchor_tag_html;
         if (obj.tags.segname_prev.length > 0)
           output ~= ".segment_prev: " ~ obj.tags.segname_prev;