Add metadata-only PPI save automation

docs/modernization/roadmap.md

@@ -340,6 +340,8 @@ asset-level decode coverage, and
 `pano_cli load-project` creates a `pp_document` projection with per-layer frame
 counts, durations, and decoded face-pixel payload attachment when PPI image
 payloads are present.
+`pano_cli save-project` writes a metadata-only one-layer/one-frame PPI through
+the extracted `pp_assets` writer and round-trips it through `load-project`.
 `pano_cli create-document` can create simple animation documents with explicit
 frame count/duration. `pano_cli simulate-document-edits` exercises pure
 layer metadata, frame reordering, active-index preservation, and tiny
@@ -713,6 +715,9 @@ Results:
 - `pano_cli_load_project_metadata_smoke` passed and reports a `pp_document`
   projection with per-layer frame counts, durations, and zero loaded face
   payloads for the minimal PPI fixture.
+- `pano_cli_save_project_roundtrip_smoke` passed and proves the metadata-only
+  `pp_assets` PPI writer can save a generated PPI and reload it through
+  `pano_cli load-project`.
 - `pano_cli_parse_layout_smoke` passed.
 - `pano_cli_simulate_stroke_smoke` passed and reports deterministic stroke
   sample counts/distances.
@@ -777,6 +782,9 @@ Results:
   `pp_assets`, attaches them to a pure `pp_document` face payload, and has a
   truncated-PNG rejection smoke test. A checked-in decodable image fixture and
   full export/round-trip automation remain future `pano_cli` tasks.
+- `pano_cli save-project` exposes a metadata-only PPI writer through JSON
+  automation and is covered by a save/load round-trip smoke test. Full
+  dirty-face payload save parity remains tracked by DEBT-0013.
 - PowerShell package-smoke wrapper validates the Windows CMake app executable
   and runtime `data/` copy.
 - Android arm64 configured with NDK 29.0.14206865 through the platform-build