Move export snapshot platform support to app core

docs/modernization/roadmap.md

@@ -733,7 +733,10 @@ retained fallback. `pp_app_core` now owns the document-snapshot export route
 decision used by those live adapters, covering platform support, target support,
 PNG/JPEG equirectangular target support, collection/cube target support, and
 incomplete renderer-payload fallback; `pano_cli plan-export-snapshot-route`
-exposes the same decision for automation, including unsupported target paths.
+exposes the same decision for automation, including unsupported target paths
+and unsupported platform fallback. The retained export bridge now consumes the
+app-core current-platform policy for snapshot-writer attempts instead of owning
+local Web writer gates.
 Web handoff, video, depth writer replacement, and incomplete-readback cases
 still delegate to retained `Canvas` writers after route/readiness reporting.
 Depth export now also plans the retained image/depth file targets in
@@ -2305,7 +2308,9 @@ Results:
   equirectangular, layer, animation-frame, and cube-face writers, including
   PNG/JPEG target support, collection/cube target support, platform support,
   and incomplete renderer-payload fallback reasons. `pano_cli
-  plan-export-snapshot-route` exposes the same policy as JSON.
+  plan-export-snapshot-route` exposes the same policy as JSON, and the live
+  retained export bridge consumes the app-core current-platform support helper
+  instead of local Web writer gates.
 - `pp_app_core_document_import_tests` passed, covering wide equirectangular,
   legacy vertical cube strip, regular transform-placement, and invalid-dimension
   import route decisions, equirectangular service dispatch, transform import