Modernize retained Linux WebGL CMake baselines

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