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>

.gitattributes

@@ -2,7 +2,6 @@
 * text=auto eol=lf
 
 # Vendor: do not count toward language stats
-lib/Plutonium/** linguist-vendored=true
 deps/**          linguist-vendored=true
 
 # Binary assets