Linux/Windows: REALPATH-resolve CMake plugin location before reaching into src/native_bridge/

Flutter symlinks the plugin into <example>/<plat>/flutter/ephemeral/.plugin_symlinks/.
CMake follows the symlink to find the CMakeLists.txt but resolves relative
add_subdirectory paths against the literal symlink directory, so
add_subdirectory("../../native_bridge") lands at the wrong path
(<example>/<plat>/flutter/ephemeral/.plugin_symlinks/native_bridge — doesn't exist).
Use get_filename_component(... REALPATH) to canonicalize before joining the path.

Android isn't affected — gradle doesn't use the .plugin_symlinks mechanism.

Author: FeodorFitsner

src/serious_python_bridge/linux/CMakeLists.txt

@@ -3,7 +3,15 @@ cmake_minimum_required(VERSION 3.10)
 set(PROJECT_NAME "serious_python_bridge")
 project(${PROJECT_NAME} LANGUAGES C)
 
-add_subdirectory("../../native_bridge" "${CMAKE_CURRENT_BINARY_DIR}/shared")
+# Flutter symlinks the plugin into <example>/linux/flutter/ephemeral/
+# .plugin_symlinks/serious_python_bridge — CMake follows the symlink to find
+# this file but resolves relative add_subdirectory paths against the literal
+# symlink location, not the canonical source. REALPATH-resolve first so
+# ../../native_bridge lands at <repo>/src/native_bridge regardless of how
+# Flutter staged the plugin.
+get_filename_component(_real_list_dir "${CMAKE_CURRENT_LIST_DIR}" REALPATH)
+get_filename_component(_native_bridge_dir "${_real_list_dir}/../../native_bridge" REALPATH)
+add_subdirectory("${_native_bridge_dir}" "${CMAKE_CURRENT_BINARY_DIR}/shared")
 
 # Flutter picks up bundled libraries via this variable and copies the file
 # alongside the app binary; Dart resolves it via DynamicLibrary.open.

src/serious_python_bridge/windows/CMakeLists.txt

@@ -39,7 +39,15 @@ set(FLET_BRIDGE_PYTHON_LIBRARIES
   "${PYTHON_DOWNLOAD_DIR}/libs/python3$<$<CONFIG:Debug>:_d>.lib"
 )
 
-add_subdirectory("../../native_bridge" "${CMAKE_CURRENT_BINARY_DIR}/shared")
+# Flutter symlinks the plugin into <example>/windows/flutter/ephemeral/
+# .plugin_symlinks/serious_python_bridge — CMake follows the symlink to find
+# this file but resolves relative add_subdirectory paths against the literal
+# symlink location, not the canonical source. REALPATH-resolve first so
+# ../../native_bridge lands at <repo>/src/native_bridge regardless of how
+# Flutter staged the plugin.
+get_filename_component(_real_list_dir "${CMAKE_CURRENT_LIST_DIR}" REALPATH)
+get_filename_component(_native_bridge_dir "${_real_list_dir}/../../native_bridge" REALPATH)
+add_subdirectory("${_native_bridge_dir}" "${CMAKE_CURRENT_BINARY_DIR}/shared")
 
 # Make python-windows-for-dart's libs/ visible to MSVC's auto-link. pyconfig.h
 # emits `#pragma comment(lib, "pythonXY(_d).lib")` and MSVC's default LIB path