Split Linux platform target and move runtime/platform state

docs/modernization/tasks.md

@@ -41,8 +41,8 @@ Completed, blocked, and superseded task history moved to
   `src/node_stroke_preview.cpp`, `src/app.cpp`, `src/app_dialogs.cpp`, and the
   extracted canvas/platform containment files.
 - The platform boundary is not finished:
-  - `pp_platform_api` no longer compiles Apple implementation files, but it
-    still compiles concrete Linux platform sources
+  - `pp_platform_api` no longer compiles Apple or Linux implementation files,
+    but broader concrete platform ownership is still not fully separated
   - `platform_apple` no longer reaches `App::I` directly, and Linux FPS title
     reporting now uses an injected callback, but retained Apple bridging and
     broader platform-to-app singleton reach are still open in
@@ -770,6 +770,9 @@ Current slice:
 - the remaining Windows app shell in `main(...)` now also routes through
   `run_main_application(...)` in `src/platform_windows/windows_runtime_shell.*`,
   which reduces `src/main.cpp` to a minimal `main`/`WinMain` wrapper
+- the retained Win32 main-thread task queue now also lives in `AppRuntime`
+  instead of `src/platform_windows/windows_platform_services.cpp`, which
+  removes another runtime queue from retained platform-local static state
 - prepared-file background work now runs through an `AppRuntime`-owned worker
   queue instead of a retained static worker in `src/app_events.cpp`
 - canvas async import/export/save/open background work now also runs through an
@@ -1077,8 +1080,13 @@ Current slice:
 - Apple concrete platform code now lives in the new `pp_platform_apple`
   target, and `panopainter_app` plus `pp_platform_api_tests` now link that
   concrete target where needed.
-- The dependency direction is cleaner for Apple, but the same split is still
-  incomplete for Linux and the broader concrete platform family.
+- `pp_platform_api` now also stops compiling
+  `src/platform_linux/linux_platform_services.*`.
+- Linux concrete platform code now lives in the new `pp_platform_linux`
+  target, and `pp_legacy_app` plus `pp_platform_api_tests` now link that
+  concrete target where needed.
+- The dependency direction is cleaner for Apple and Linux, but the broader
+  concrete platform family is still being separated.
 
 Write scope:
 - `CMakeLists.txt`
@@ -1093,8 +1101,8 @@ Read scope:
 Done when:
 - `pp_platform_api` contains only platform-neutral interfaces, policies, and
   shared helpers.
-- Apple implementation files are built by a concrete platform target instead of
-  the API target.
+- Apple and Linux implementation files are built by concrete platform targets
+  instead of the API target.
 - The dependency direction is obvious from CMake without reading debt notes.
 
 Mini-model packet:
@@ -1145,6 +1153,10 @@ Current slice:
   snapshots without direct `App::I` reads in the touched paths
 - retained Apple callback injection and broader `platform_legacy` singleton
   reach are still open
+- the `platform_legacy`-mirrored Apple/GLFW handle cluster is now seeded
+  explicitly from the iOS, macOS, Linux, and WebGL entrypoints through
+  `src/platform_legacy/legacy_platform_state.*` instead of being copied out
+  of `App`
 - The remaining Win32 shell wrappers for close, async lock/swap,
   stylus/FPS updates, VR start/stop, window-state save, and the
   window-handle accessor now live in
@@ -1188,8 +1200,12 @@ Current slice:
   `read_platform_vr_session_snapshot()` in
   `src/platform_windows/windows_platform_services.*`, with app-side reads now
   routed through `App::vr_session_snapshot()`.
-- `App` still owns other platform-facing handles and retained legacy platform
-  state is not fully removed, so this remains a live ownership task.
+- The `platform_legacy`-mirrored Apple/GLFW handle cluster also no longer
+  lives on `App`; retained Apple/GLFW platform state is now seeded explicitly
+  from the iOS, macOS, Linux, and WebGL entrypoints through
+  `src/platform_legacy/legacy_platform_state.*`.
+- `App` still owns Android-native handles plus broader retained legacy
+  platform state, so this remains a live ownership task.
 
 Write scope:
 - `src/platform_legacy/legacy_platform_services.*`