build: migrate Plutonium to ExternalProject_Add + IMPORTED target

Replaces add_custom_command/add_custom_target with ExternalProject_Add,
which correctly declares BUILD_BYPRODUCTS for Ninja and propagates include
paths via INTERFACE_INCLUDE_DIRECTORIES on the IMPORTED STATIC target.

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

CMakeLists.txt

@@ -35,10 +35,7 @@ file(GLOB_RECURSE NXST_SOURCES
 add_executable(NXST ${NXST_SOURCES})
 
 # ── Include paths ─────────────────────────────────────────────────────────────
-target_include_directories(NXST PRIVATE
-    include
-    lib/Plutonium/include
-)
+target_include_directories(NXST PRIVATE include)
 
 # ── pkg-config (uses aarch64-none-elf-pkg-config set by Switch.cmake) ─────────
 find_package(PkgConfig REQUIRED)
@@ -52,25 +49,33 @@ pkg_check_modules(PORTLIBS REQUIRED IMPORTED_TARGET ${NXST_PKG_MODULES})
 
 target_include_directories(NXST PRIVATE ${PORTLIBS_INCLUDE_DIRS})
 
-# ── Link libraries ────────────────────────────────────────────────────────────
-# ── Plutonium static library ──────────────────────────────────────────────────
+# ── Plutonium ─────────────────────────────────────────────────────────────────
+include(ExternalProject)
+
 set(LIBPU_A ${CMAKE_SOURCE_DIR}/lib/Plutonium/lib/libpu.a)
 
-add_custom_command(
-    OUTPUT  ${LIBPU_A}
-    COMMAND make -C ${CMAKE_SOURCE_DIR}/lib/Plutonium -j
-    COMMENT "Building Plutonium (libpu.a)"
+ExternalProject_Add(plutonium_ep
+    SOURCE_DIR      ${CMAKE_SOURCE_DIR}/lib/Plutonium
+    CONFIGURE_COMMAND ""
+    BUILD_COMMAND   make -j -C ${CMAKE_SOURCE_DIR}/lib/Plutonium
+    INSTALL_COMMAND ""
+    BUILD_IN_SOURCE 1
+    BUILD_BYPRODUCTS ${LIBPU_A}
+)
+
+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
 )
-add_custom_target(plutonium DEPENDS ${LIBPU_A})
-add_dependencies(NXST plutonium)
 
 # ── Link libraries ────────────────────────────────────────────────────────────
 # Order matters for static linking: put most dependent libs first.
-# libpu.a first (contains C wrappers not in Plutonium source).
 # drm_nouveau, harfbuzz, freetype, z appended explicitly after pkg-config output
 # to fix the freetype→harfbuzz static link order (see build notes from libnx update).
 target_link_libraries(NXST PRIVATE
-    ${LIBPU_A}
+    plutonium
     PkgConfig::PORTLIBS
     drm_nouveau
     harfbuzz