Modernize retained Linux WebGL CMake baselines

docs/modernization/build-inventory.md

@@ -17,8 +17,8 @@ Keep it updated as platform paths move to shared CMake targets.
 | Android standard | `android/android/build.gradle`, `android/android/CMakeLists.txt` | Retained native library target `native-lib`; CMake 3.10/C++23 baseline now links the standard arm64 package path with modern component/service sources and the generated `nanort` overlay helper |
 | Android Quest | `android/quest/build.gradle`, `android/quest/CMakeLists.txt` | OVR SDK imported libraries; CMake 3.10/C++23 baseline and current Yoga source list configure with the shared Android package compatibility helper |
 | Android Focus/Wave | `android/focus/build.gradle`, `android/focus/CMakeLists.txt` | Wave SDK imported libraries; CMake 3.10/C++23 baseline and current Yoga source list configure with the shared Android package compatibility helper |
-| Linux | `linux/CMakeLists.txt` | Old CMake 3.4, C++14 flag |
-| WebGL/Emscripten | `webgl/CMakeLists.txt` | Old CMake 3.4, WebGL2 flags |
+| Linux | `linux/CMakeLists.txt` | Retained app target now uses CMake 3.10 and target-level C++23 while package/root target migration remains open |
+| WebGL/Emscripten | `webgl/CMakeLists.txt` | Retained WebGL app target now uses CMake 3.10 and target-level C++23 with retained WebGL2/Emscripten link flags |
 
 ## Existing Version Generation
 
@@ -211,6 +211,11 @@ powershell -ExecutionPolicy Bypass -File scripts\automation\apple-remote-build.p
   `panopainter_platform_build_apple_remote`. These targets call the existing
   platform automation scripts from CMake and keep platform validation discoverable
   from the CMake target graph while app/package target migration remains open.
+- Retained Linux and WebGL app CMake entrypoints are still separate from root
+  CMake, but now share the modernization baseline of CMake 3.10 and
+  target-level `cxx_std_23`. `panopainter_retained_platform_cmake_self_test`
+  guards those baselines until these entrypoints are replaced by shared root
+  CMake package/app targets.
 - Root CMake exposes named `fuzz` and `stress` CTest presets. `fuzz` currently
   runs deterministic parser/serializer edge tests for binary streams, image
   metadata, PPI, stroke scripts, and layout XML; `stress` currently runs the

docs/modernization/roadmap.md

@@ -1412,6 +1412,11 @@ checks: `panopainter_package_readiness`,
 `panopainter_android_native_package_smoke`. App/package entrypoints still need
 to consume shared targets and remain covered by debt until package validation
 is migrated from legacy package projects to root CMake.
+Retained Linux and WebGL app CMake entrypoints now match the interim platform
+baseline used by Android package paths: CMake 3.10 plus target-level
+`cxx_std_23`, with `panopainter_retained_platform_cmake_self_test` guarding
+against regressions while those entrypoints wait behind root CMake package/app
+target migration.
 Apple compile validation now runs on the local Mac mini SSH host
 `panopainter-mac` through `scripts/automation/apple-remote-build.ps1`. The host
 uses Homebrew CMake/Ninja/Git plus full Xcode via `DEVELOPER_DIR`, pulls the
@@ -2413,6 +2418,11 @@ Results:
   panopainter_windows_app_package_smoke`, which builds the CMake `PanoPainter`
   app target, validates the executable/runtime `data/` copy, and reports the
   still-blocked Windows AppX package state.
+- Retained Linux and WebGL app CMake files now use CMake 3.10 and target-level
+  C++23 instead of global C++14 flags; `python
+  scripts/dev/check_retained_platform_cmake.py` and CTest
+  `panopainter_retained_platform_cmake_self_test` guard those baselines while
+  the actual Linux/WebGL app/package targets remain outside root CMake.
 - Root CMake now exposes platform validation targets for the default headless
   platform-build sweep, the Android standard/Quest/Focus root CMake asset
   component sweep, the vcpkg-backed UI core dependency boundary, and the