Add document PPI export boundary

docs/modernization/build-inventory.md

@@ -93,7 +93,7 @@ Known local toolchain state:
   `pp_ui_core`, `pano_cli`, and their current headless test binaries,
   including foundation event/logging/task queue coverage, PNG metadata and
   decode, PPI header/layout, settings document, document
-  snapshot/per-layer-frame/move/duration/face-pixel coverage, paint
+  snapshot/per-layer-frame/move/duration/face-pixel/PPI export coverage, paint
   brush/stroke/stroke-script coverage, renderer shader descriptor and OpenGL
   capability coverage, UI color parsing, and layout XML parse coverage.
 - `pano_cli inspect-image` reports PNG IHDR metadata as JSON and is covered by
@@ -116,6 +116,9 @@ Known local toolchain state:
 - `pp_document_ppi_import_tests` attaches decoded PPI dirty-face payloads to
   `pp_document` layer/frame storage and rejects payloads outside document
   layers.
+- `pp_document_ppi_export_tests` exports pure `pp_document` metadata,
+  per-layer frame durations, and RGBA8 face payloads to PPI bytes through
+  `pp_assets`, then decodes and reimports them for round-trip coverage.
 - `pano_cli load-project` creates a `pp_document` projection with per-layer
   frame counts, durations, and decoded face-pixel payloads when present; the
   metadata-only minimal fixture remains covered by
@@ -294,6 +297,9 @@ Known local toolchain state:
   through JSON automation and is covered by metadata-only and
   dirty-face-payload round-trip smoke tests; full legacy canvas save parity
   remains tracked by DEBT-0013.
+- `pp_document::export_ppi_project_document` exposes pure document-to-PPI byte
+  export through CTest coverage; legacy Canvas save integration remains a
+  future DEBT-0010/DEBT-0013 task.
 - `pp_ui_core` consumes vcpkg tinyxml2 only when `PP_USE_VCPKG_TINYXML2=ON`
   through the vcpkg preset; default and Android validation still use the
   retained vendored fallback tracked by DEBT-0012.