e.g. /var/tmp/portage/dev-java/zstd-jni-1.5.2.4/work/zstd-jni-37faee49643b2739457716e3752f44fcaa09005f/src/main/native/compress/zstd_lazy.c is clearly from app-arch/zstd.
There are files in zstd-jni not provided by app-arch/zstd: src/main/native/jni_zstd.c src/main/native/jni_zdict.c src/main/native/jni_outputstream_zstd.c src/main/native/jni_inputstream_zstd.c src/main/native/jni_fast_zstd.c src/main/native/jni_directbufferdecompress_zstd.c src/main/native/jni_directbuffercompress_zstd.c