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

.gitignore

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

.gitmodules

@@ -1,3 +0,0 @@
-[submodule "lib"]
-	path = lib
-	url = https://github.com/XorTroll/Plutonium.git

CMakeLists.txt

@@ -52,12 +52,16 @@ target_include_directories(NXST PRIVATE ${PORTLIBS_INCLUDE_DIRS})
 # ── Plutonium ─────────────────────────────────────────────────────────────────
 include(ExternalProject)
 
-set(LIBPU_A ${CMAKE_SOURCE_DIR}/lib/Plutonium/lib/libpu.a)
+set(PLUTONIUM_SOURCE_DIR ${CMAKE_BINARY_DIR}/plutonium)
+set(LIBPU_A ${PLUTONIUM_SOURCE_DIR}/Plutonium/lib/libpu.a)
 
 ExternalProject_Add(plutonium_ep
-    SOURCE_DIR      ${CMAKE_SOURCE_DIR}/lib/Plutonium
+    GIT_REPOSITORY  https://github.com/XorTroll/Plutonium.git
+    GIT_TAG         b56564b70d038c59eef875f2c3cf436859c827f2
+    GIT_PROGRESS    ON
+    SOURCE_DIR      ${PLUTONIUM_SOURCE_DIR}
     CONFIGURE_COMMAND ""
-    BUILD_COMMAND   make -j -C ${CMAKE_SOURCE_DIR}/lib/Plutonium
+    BUILD_COMMAND   make -j -C ${PLUTONIUM_SOURCE_DIR}/Plutonium
     INSTALL_COMMAND ""
     BUILD_IN_SOURCE 1
     BUILD_BYPRODUCTS ${LIBPU_A}
@@ -67,8 +71,8 @@ add_library(plutonium STATIC IMPORTED GLOBAL)
 add_dependencies(plutonium plutonium_ep)
 set_target_properties(plutonium PROPERTIES
     IMPORTED_LOCATION ${LIBPU_A}
-    INTERFACE_INCLUDE_DIRECTORIES ${CMAKE_SOURCE_DIR}/lib/Plutonium/include
 )
+target_include_directories(NXST PRIVATE ${PLUTONIUM_SOURCE_DIR}/Plutonium/include)
 
 # ── Link libraries ────────────────────────────────────────────────────────────
 # Order matters for static linking: put most dependent libs first.