refactor: make NativeCall.requireNative return void (#15)

Every caller uses it as a guard statement and discards the returned
segment, so the fluent return was dead. Drop it.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: dfa1

zstd/src/main/java/io/github/dfa1/zstd/NativeCall.java

@@ -59,12 +59,11 @@ private static String errorName(long code) {
     /// Guards a zero-copy entry point: the segment handed to zstd must be backed
     /// by native (off-heap) memory, since its address is dereferenced in C. Fails
     /// fast with a clear message instead of the FFM linker's cryptic error.
-    static MemorySegment requireNative(MemorySegment seg, String name) {
+    static void requireNative(MemorySegment seg, String name) {
         if (!seg.isNative()) {
             throw new IllegalArgumentException(
                     name + " must be a native (off-heap) MemorySegment; got a heap segment");
         }
-        return seg;
     }
 
     /// Rethrows any `Throwable` as if unchecked, laundering the checked