build: replace Plutonium submodule with ExternalProject_Add fetch

Plutonium is now downloaded at build time via ExternalProject_Add
(GIT_REPOSITORY + GIT_TAG) instead of being a git submodule.
Removes lib/, .gitmodules, and the linguist-vendored entry for it.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitignore

@@ -49,3 +49,4 @@ CMakeCache.txt
 CMakeFiles/
 cmake_install.cmake
 compile_commands.json
+.cache/