**`PluginManager.cpp`**: Replace `LoadLibraryW` / `FreeLibrary` / `GetProcAddress` (Windows-only) with `dlopen` / `dlclose` / `dlsym` on Linux/macOS, guarded by `TE_PLATFORM_*` defines.

[github-actions]

Part of milestone [Long-Term] 4. Cross-Platform Build & Runtime Support.
Tracks: PluginManager.cpp: Replace LoadLibraryW / FreeLibrary / GetProcAddress (Windows-only) with dlopen / dlclose / dlsym on Linux/macOS, guarded by TE_PLATFORM_* defines.