refactor: replace vendored xxhash.h with CMake FetchContent dependency

Remove 7490-line vendored xxhash.h, use FetchContent to pull xxHash v0.8.3
at CMake configure time (SOURCE_SUBDIR cmake_unofficial). Links as static
library via the xxhash target.

src/shared/delta.c

@@ -5,7 +5,7 @@
 
 #define XXH_STATIC_LINKING_ONLY
 #define XXH_IMPLEMENTATION
-#include "xxhash.h"
+#include <xxhash.h>
 
 uint32_t delta_adler32(const void *data, uint32_t len) {
   const uint8_t *p = (const uint8_t *)data;