Add java.nio.ByteBuffer overloads for compress/decompress

[dfa1]

Motivation

NIO-based callers (channels, networking, mmap via FileChannel.map) hold data in ByteBuffer. Today they must extract a byte[] to use the API. A direct ByteBuffer is off-heap and can be wrapped as a MemorySegment with no copy — a natural fit for this binding's zero-copy story.

Proposal

Add ByteBuffer overloads (on Zstd, or a small dedicated entry point):

public static ByteBuffer compress(ByteBuffer src, int level);
public static ByteBuffer decompress(ByteBuffer src, int maxSize);

• For a direct buffer: wrap MemorySegment.ofBuffer(src) — zero copy in.
• For a heap buffer: fall back to the array path.
• Respect position()/limit(); advance position like other NIO ops.

Acceptance criteria

• Both direct and heap buffers handled; position/limit semantics documented and tested.
• /// javadoc per CLAUDE.md; javadoc build clean.
• Tests cover direct + heap, empty, and a round-trip against the existing byte[] path.

Notes

See docs/zero-copy.md for the segment-first philosophy this should follow.

Pointers

• zstd/src/main/java/io/github/dfa1/zstd/Zstd.java