Delete orphaned legacy platform shim

docs/modernization/debt.md

@@ -18,6 +18,11 @@ agent or engineer to remove them without reconstructing context from chat.
 
 ## Reductions
 
+- 2026-06-17: `DEBT-0017` was narrowed again. The orphaned
+  `src/platform_legacy/legacy_platform_services.*` shim and its
+  `legacy_platform_fallback_behavior.h` helper were deleted after the root app,
+  WebGL entrypoint, and retained Android package graphs all moved off that
+  fallback path.
 - 2026-06-17: `DEBT-0017` was narrowed again. The retained Android package
   CMake projects now compile `src/platform_android/android_platform_services.cpp`
   instead of `src/platform_legacy/legacy_platform_services.cpp`, matching the

docs/modernization/roadmap.md

@@ -70,6 +70,12 @@ What is already real:
 - `pp_app_core`
 
 Latest slice:
+- The orphaned `src/platform_legacy/legacy_platform_services.*` shim and its
+  `src/platform_legacy/legacy_platform_fallback_behavior.h` helper are now
+  deleted from the repo.
+- The root app graph, WebGL entrypoint, and retained Android package graphs had
+  already moved off that fallback path, so the old generic platform-legacy
+  adapter no longer exists as a compiled code path.
 - `android/android/CMakeLists.txt`, `android/quest/CMakeLists.txt`, and
   `android/focus/CMakeLists.txt` now compile
   `src/platform_android/android_platform_services.cpp` instead of

docs/modernization/tasks.md

@@ -84,6 +84,12 @@ Completed, blocked, and superseded task history moved to
   the queue is now ordered by code movement instead.
 
 Current slice:
+- The orphaned `src/platform_legacy/legacy_platform_services.*` shim and
+  `src/platform_legacy/legacy_platform_fallback_behavior.h` helper are now
+  deleted.
+- Root builds, the WebGL entrypoint, and the retained Android package graphs
+  had already moved off that fallback path, so the old generic legacy platform
+  adapter is no longer present as a compiled code path.
 - `android/android/CMakeLists.txt`, `android/quest/CMakeLists.txt`, and
   `android/focus/CMakeLists.txt` now compile
   `src/platform_android/android_platform_services.cpp` instead of the retained