Add document export file automation

docs/modernization/build-inventory.md

@@ -123,6 +123,9 @@ Known local toolchain state:
 - `pano_cli simulate-document-export` exposes the same pure document-to-PPI
   export, asset-level decode, and document reimport path through JSON
   automation and is covered by `pano_cli_simulate_document_export_smoke`.
+- `pano_cli save-document-project` writes that pure document export to a PPI
+  file and is covered by `pano_cli_save_document_project_roundtrip_smoke`,
+  which inspects and loads the generated file.
 - `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
@@ -306,6 +309,8 @@ Known local toolchain state:
   future DEBT-0010/DEBT-0013 task.
 - `pano_cli simulate-document-export` exposes document export round-trip state
   through JSON automation for agent-driven checks.
+- `pano_cli save-document-project` exposes file-writing document export
+  automation for inspect/load round trips.
 - `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.