Plan display file actions in app core

docs/modernization/roadmap.md

@@ -454,6 +454,9 @@ platform branches execute.
 `pano_cli plan-picked-path` exposes the app-core selected-path decision used by
 live image, file, save-file, and directory picker branches before retained
 platform callbacks or legacy picker bridges continue.
+`pano_cli plan-display-file` exposes the app-core external file presentation
+decision used by live display-file requests before retained platform open-file
+bridges continue.
 `pano_cli plan-cloud-upload` exposes the app-core cloud upload decision used by
 the live cloud upload command for missing-canvas, new-document warning, publish
 prompt, and dirty-document save-before-upload states before legacy UI, canvas,
@@ -942,10 +945,14 @@ Results:
   decisions as JSON.
 - `pp_app_core_document_platform_io_tests` passed, covering empty selected-path
   filtering and non-empty picked-path callback planning before platform picker
-  callbacks.
+  callbacks, plus empty/non-empty display-file planning before platform
+  display callbacks.
 - `pano_cli_plan_picked_path_empty_smoke` and
   `pano_cli_plan_picked_path_selected_smoke` passed and expose app-core picker
   selected-path decisions as JSON.
+- `pano_cli_plan_display_file_empty_smoke` and
+  `pano_cli_plan_display_file_selected_smoke` passed and expose app-core
+  display-file decisions as JSON.
 - `panopainter_validate_shaders` passed, validating 25 shader programs and 7
   shader includes for stage markers and include graph integrity.
 - `pp_renderer_gl_capabilities_tests` passed on default MSVC, vcpkg-headless,