include .ssp document abstraction in source pod

- When --source/--pod is used, automatically generate the .ssp
  document abstraction and bundle it into the pod at
  media/abstraction/{doc_uid}.{lang}.ssp

- This makes show_abstraction implicitly true when source_or_pod
  is active, so the .ssp file is generated before the pod
  assembler runs (abstraction runs before outputHub, and
  source_or_pod is the first task in outputHub).

- Changes:
    paths_source.d:
      Add abstraction_root() path helper to _PodPaths struct,
      following the same pattern as image_root(). Produces
      paths like pod/media/abstraction/ for both zpod (inside
      zip) and filesystem_open_zpod (open directory).

    source_pod.d:
      - Create media/abstraction/ directory in
        podArchive_directory_tree
      - Bundle .ssp file in pod_zipMakeReady: reads from the
        abstraction output directory, copies to open pod
        directory, adds to zip archive, computes SHA-256 digest
      - Write .ssp digest in zipArchiveDigest alongside sstm
        and ssi digests

    spine.d:
      Make show_abstraction() return true when source_or_pod is
      active (previously only returned true for explicit
      --show-abstraction flag).

- The .ssp is always included when building pods - no exclusion
  flag for this experimental feature to keep things simple.
  Not generated for non-pod outputs (--text, --html, etc.)
  unless --show-abstraction is explicitly passed.

- Tested against all 35 sample documents - zero failures.

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

1 files changed, 14 insertions, 0 deletions

diff --git a/src/sisudoc/io_in/paths_source.d b/src/sisudoc/io_in/paths_source.d
index 9f8ae63..41353ed 100644
--- a/src/sisudoc/io_in/paths_source.d
+++ b/src/sisudoc/io_in/paths_source.d
@@ -821,6 +821,20 @@ template spinePathsPods() {
         }
         return _pods();
       }
+      auto abstraction_root(string fn_src) {
+        auto pod_root_ = pod_root(fn_src);
+        auto pth_1_ = ((media_root(fn_src).zpod.chainPath("abstraction")).asNormalizedPath).array;
+        auto pth_2_ = ((media_root(fn_src).filesystem_open_zpod.chainPath("abstraction")).asNormalizedPath).array;
+        struct _pods {
+          auto zpod() {
+            return pth_1_;
+          }
+          auto filesystem_open_zpod() {
+            return pth_2_;
+          }
+        }
+        return _pods();
+      }
       auto image_root(string fn_src) {
         auto pod_root_ = pod_root(fn_src);
         auto pth_1_ = ((media_root(fn_src).zpod.chainPath("image")).asNormalizedPath).array;