refactor: drive ZstdStreamBuffer offsets from a MemoryLayout

[dfa1]

What

ZstdStreamBuffer backs the native ZSTD_inBuffer / ZSTD_outBuffer structs ({ void* ptr; size_t size; size_t pos; }). It previously hardcoded the struct shape with magic numbers:

static final long BYTES = 24;
private static final long OFF_PTR = 0;
private static final long OFF_SIZE = 8;
private static final long OFF_POS = 16;

This replaces those with a proper named StructLayout and derives everything from it:

• BYTES now comes from LAYOUT.byteSize() (same name/type/visibility, still package-accessible).
• Fields are accessed through VarHandles obtained from the layout (ADDRESS for ptr, JAVA_LONG for size/pos), passing base offset 0L.
• The allocation uses the layout directly (arena.allocate(LAYOUT)).

Why

Self-documenting field names instead of byte offsets, and the offsets/size can no longer drift out of sync with the declared layout.

Behavior

Behavior-preserving. The public/package surface is unchanged (ZstdStreamBuffer(Arena), segment(), set(MemorySegment, long), size(), pos()), and set still zeroes pos.

Verification

• ./mvnw -pl zstd test — 261 tests, all green (streaming paths ZstdSegmentStreamTest / ZstdStreamTest exercise this struct).
• ./mvnw javadoc:javadoc -pl zstd — zero output.
• ./mvnw -pl zstd validate (checkstyle) — clean.

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