Split Linux platform target and move runtime/platform state

docs/modernization/roadmap.md

@@ -92,6 +92,18 @@ Current hotspot files:
 
 Latest slice:
 
+- `pp_platform_api` no longer compiles
+  `src/platform_linux/linux_platform_services.*`; Linux concrete platform code
+  now lives in `pp_platform_linux`, which `pp_legacy_app` and
+  `pp_platform_api_tests` link where needed.
+- Win32 main-thread queued task ownership now lives in `AppRuntime` instead of
+  `src/platform_windows/windows_platform_services.cpp`, which removes another
+  runtime queue from retained platform-local static state and leaves the
+  Windows shell as a thin forwarder.
+- The `platform_legacy`-mirrored Apple and GLFW handle cluster 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.*`.
 - `pp_platform_api` no longer compiles
   `src/platform_apple/apple_platform_services.*`; Apple concrete platform
   code now lives in the new `pp_platform_apple` target, and
@@ -187,8 +199,8 @@ Latest slice:
 Current architecture mismatches that must be treated as real blockers:
 
 - `pp_platform_api` no longer compiles Apple implementation files, but it
-  still owns concrete Linux platform sources instead of only platform-neutral
-  policy and interface code.
+  still owns too much concrete platform implementation instead of only
+  platform-neutral policy and interface code.
 - `src/platform_apple/apple_platform_services.cpp` no longer reaches `App::I`
   directly, and Linux FPS title reporting now uses an injected callback, but
   retained Apple bridging in `platform_legacy` and other platform/app coupling
@@ -203,7 +215,9 @@ Current architecture mismatches that must be treated as real blockers:
   the retained GLFW window hooks, Apple handle snapshots, and fallback
   storage-path return now also using local retained-state helpers instead of
   direct method-body reads, while Windows VR session snapshot state now also
-  lives behind platform-owned helpers instead of on `App`.
+  lives behind platform-owned helpers instead of on `App`, and the
+  `platform_legacy`-mirrored Apple/GLFW handle cluster is now seeded
+  explicitly from platform entrypoints instead of being copied out of `App`.
 - `src/platform_legacy/legacy_platform_services.*` is still part of the live
   app shell.
 - `pp_panopainter_ui` still depends on `pp_legacy_app`.