Own main workers and narrow Apple render hooks

docs/modernization/roadmap.md

@@ -98,15 +98,17 @@ Current architecture mismatches that must be treated as real blockers:
   directly, and Linux FPS title reporting now uses an injected callback, but
   retained Apple bridging in `platform_legacy` and other platform/app coupling
   remain, even though iOS keyboard visibility and prepared-file save handoff
-  now also route through explicit Apple bridge callbacks.
+  now also route through explicit Apple bridge callbacks and Apple render-
+  context hooks plus iOS main-render-target binding now route through the same
+  bridge style.
 - `src/platform_legacy/legacy_platform_services.*` is still part of the live
   app shell.
 - `pp_panopainter_ui` still depends on `pp_legacy_app`.
 - `Canvas`, `NodeCanvas`, and `NodeStrokePreview` still own too much live
   OpenGL execution around the renderer boundary, even though `NodeCanvas`
   display resolve, cache-to-screen composite, post-draw mask/grid/current-mode
-  sequencing, and per-layer/per-plane retained draw execution now route
-  through retained draw-merge helpers.
+  sequencing, per-layer/per-plane retained draw execution, and shared
+  checkerboard background setup now route through retained draw-merge helpers.
 - `app_layout.cpp` and `app_dialogs.cpp` are still mixed shell/controller files
   rather than thin composition/binding surfaces.
 - `App`, `Canvas`, `Node`, retained workers, and platform entrypoints still use
@@ -115,7 +117,8 @@ Current architecture mismatches that must be treated as real blockers:
   ownership, even though most previously detached or raw app-facing worker
   launches now use owned `std::jthread` or service-owned worker queues and
   `AppRuntime` now owns render/UI workers with explicit `std::jthread`
-  shutdown semantics.
+  shutdown semantics while the Windows splash-dialog and HMD renderer workers
+  also use owned `std::jthread` lifecycle.
 - Modern C++23 usage exists in extracted components, especially `std::span`,
   explicit result/status objects, and a few concepts, but the live app still
   does not consistently express ownership, thread affinity, or renderer