Add explicit PPI project writer

docs/modernization/roadmap.md

@@ -344,11 +344,11 @@ 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 generated multi-layer, multi-frame PPI files
-with layer opacity, blend mode, alpha lock, and visibility metadata through the
-extracted `pp_assets` writer and round-trips metadata-only and test
-dirty-face-payload variants through `load-project`; dirty-face payloads can be
-targeted to explicit generated layer/frame slots for animation coverage.
+`pp_assets` can write generated PPI projects with explicit per-layer names,
+visibility, opacity, blend mode, alpha lock, per-layer frame durations, and
+dirty-face payloads targeted to layer/frame/face slots. `pano_cli save-project`
+exposes the generated writer for metadata-only and test dirty-face-payload
+round-trips 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, tiny face-payload
@@ -680,9 +680,10 @@ Results:
 - `pp_assets_image_pixels_tests` passed, including RGBA8 PNG decode and corrupt
   payload rejection.
 - `pp_assets_ppi_header_tests` passed, including PPI thumbnail/body layout,
-  body summary validation, layer/frame indexing, dirty-face PNG payload
-  metadata validation, targeted layer/frame dirty-face writing, and decoded
-  dirty-face payload coverage.
+  body summary validation, layer/frame indexing, explicit per-layer metadata
+  and per-layer frame duration writing, dirty-face PNG payload metadata
+  validation, targeted layer/frame dirty-face writing, and decoded dirty-face
+  payload coverage.
 - `pp_assets_settings_document_tests` passed.
 - `pp_paint_brush_tests` passed.
 - `pp_paint_blend_tests` passed.
@@ -801,6 +802,8 @@ Results:
   through JSON automation and is covered by metadata-only and
   dirty-face-payload save/load round-trip smoke tests. Full legacy canvas save
   parity remains tracked by DEBT-0013.
+- `pp_assets::create_ppi_project` exposes the underlying generated PPI writer
+  for non-uniform layer metadata and frame-duration extraction work.
 - 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