Add targeted PPI payload automation

docs/modernization/roadmap.md

@@ -347,7 +347,8 @@ 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-payload variants through `load-project`; dirty-face payloads can be
+targeted to explicit generated layer/frame slots for animation coverage.
 `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
@@ -680,7 +681,8 @@ Results:
   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, and decoded dirty-face payload coverage.
+  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.
@@ -725,8 +727,9 @@ Results:
   `pp_assets` PPI writer can save a generated multi-frame PPI and reload it
   through `pano_cli load-project`.
 - `pano_cli_save_project_payload_roundtrip_smoke` passed and proves the
-  `pp_assets` PPI writer can save a compressed RGBA PNG dirty-face payload and
-  reload it as decoded `pp_document` face-pixel data.
+  `pp_assets` PPI writer can save a compressed RGBA PNG dirty-face payload to
+  an explicit layer/frame slot, inspect the serialized descriptor, and reload
+  it as decoded `pp_document` face-pixel data.
 - `pano_cli_parse_layout_smoke` passed.
 - `pano_cli_simulate_stroke_smoke` passed and reports deterministic stroke
   sample counts/distances.
@@ -794,9 +797,10 @@ Results:
   and has a save/import round-trip smoke test. Full legacy canvas export
   remains a future `pano_cli` task.
 - `pano_cli save-project` exposes generated multi-layer, multi-frame PPI
-  writing with layer metadata 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.
+  writing with layer metadata and targeted dirty-face layer/frame payloads
+  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.
 - 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