Write cube exports from document snapshots

docs/modernization/build-inventory.md

@@ -281,9 +281,11 @@ powershell -ExecutionPolicy Bypass -File scripts\automation\apple-remote-build.p
   snapshot smoke.
 - Live equirectangular, layer, animation-frame, and cube-face export adapters
   now prepare and log the same payload-bearing canvas document snapshot plus
-  shared renderer-neutral active-frame upload and face-PNG export reports before
-  delegating to retained `Canvas` export execution. Depth and video export
-  remain on the older retained path.
+  shared renderer-neutral active-frame upload and face-PNG export reports.
+  Cube-face export writes the pure document/renderer PNG bytes to the retained
+  face filename set and falls back to `Canvas::export_cube_faces` if snapshot
+  capture, PNG generation, or file writing fails. Equirectangular, layer,
+  animation-frame, depth, and video export remain on retained writer paths.
 - `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.

docs/modernization/capability-map.md

@@ -26,7 +26,7 @@ and validation command.
 | PNG/JPEG import | `Image`, `Canvas` import paths | `pp_assets`, `pp_document` | Fixture import, malformed file |
 | PNG/JPEG export | `Canvas`, `Image`, export dialogs | `pp_assets`, `pp_paint_renderer`, `pp_app_core` | Golden output tolerance, export start/target planning tests, live export-adapter document snapshot readiness |
 | Equirectangular import/export | `Canvas`, shaders, RTT, export dialogs | `pp_paint_renderer`, `pp_app_core` | Tiny cube/equirect golden, app-core file target tests, live export-adapter renderer-upload readiness |
-| Cube face export | `Canvas` | `pp_paint_renderer` | Pure six-face document frame composite, renderer texture-upload bridge, pure face-PNG export helper, payload-complete canvas-snapshot renderer-upload and face-PNG automation, live export-adapter renderer/export readiness, OpenGL command-plan coverage, six-face golden set |
+| Cube face export | `Canvas` fallback | `pp_paint_renderer` | Pure six-face document frame composite, renderer texture-upload bridge, pure face-PNG export helper, payload-complete canvas-snapshot renderer-upload and face-PNG automation, live document/renderer face-PNG writer with retained Canvas fallback, OpenGL command-plan coverage, six-face golden set |
 | Depth export | `Canvas`, grid tools | `pp_paint_renderer` | Float/readback validation |
 
 ## Brush And Painting

docs/modernization/roadmap.md

@@ -687,9 +687,11 @@ renderer-owned readback remain under
 `DEBT-0010`/`DEBT-0013`/`DEBT-0036`.
 Live equirectangular, layer, animation-frame, and cube-face export adapters now
 prepare the same payload-bearing document snapshot and renderer-neutral upload
-report helper plus pure face-PNG export report before delegating to retained
-`Canvas` export execution, so export workflows consume the shared renderer
-boundary without changing file output yet.
+report helper plus pure face-PNG export report. Cube-face export writes those
+document/renderer-owned PNG bytes to the legacy face filenames when available
+and falls back to retained `Canvas::export_cube_faces` on snapshot/write failure;
+the other export workflows still delegate to retained `Canvas` writers after
+readiness reporting.
 `pano_cli plan-image-import` exposes app-core planning for File > Import image
 route decisions, including wide equirectangular images, legacy vertical cube
 strips, regular transform-placement images, and invalid image dimensions; live
@@ -2528,9 +2530,11 @@ Results:
   live canvas export/save writer replacement.
 - Live image/collection/cube export adapters now prepare and log the same
   document/canvas plus shared renderer-upload and face-PNG export readiness
-  reports before retained `Canvas` export calls. Depth and video export remain
-  on their prior retained paths; actual image/cube writer replacement remains
-  tracked under export debt.
+  reports. Cube-face export now writes the pure document/renderer PNG bytes to
+  the legacy face filenames before falling back to retained `Canvas` execution
+  on failure. Equirectangular, layer, animation-frame, depth, and video export
+  remain on their prior retained writer paths; actual broader writer replacement
+  remains tracked under export debt.
 - Snapshot creation now rejects invalid embedded RGBA8 face payloads before
   document export or history can persist malformed state.
 - Package-smoke wrappers validate the Windows CMake app executable/runtime