feat: ranged byte[] compress/decompress overloads

[dfa1]

What

Adds (offset, length) sub-range overloads to io.github.dfa1.zstd.Zstd so callers holding a payload inside a larger buffer can compress/decompress it without copying the sub-range out first:

public static byte[] compress(byte[] src, int offset, int length);
public static byte[] compress(byte[] src, int offset, int length, int level);
public static byte[] decompress(byte[] compressed, int offset, int length, int maxSize);

How

• Range is validated with Objects.checkFromIndexSize(offset, length, src.length) (after a null-check) before any native memory is touched — throws IndexOutOfBoundsException with a clear message, handling negatives/overflow.
• A new ranged private helper copyIn(Arena, byte[], int offset, int length) backs the native copy; the existing whole-array copyIn delegates to it.
• The whole-array public overloads now delegate to the ranged ones (compress(src, level) -> compress(src, 0, src.length, level), etc.), keeping the existing null-check semantics identical and avoiding duplication.
• Only (long) length is passed as the native srcSize.

Tests

New RangedOverloads nested class in ZstdTest: ranged round-trips across the shared payload spread, ranged-equals-extracted-copy proof across all levels, full-range equals whole-array overload, empty range, embedded-frame ranged decompress, and negatives (negative offset/length, range past end, null src) asserting the thrown type.

Verification

• ./mvnw -pl zstd test — green (285 tests).
• ./mvnw javadoc:javadoc -pl zstd — zero output (failOnWarnings on).
• ./mvnw -pl zstd validate (checkstyle) — clean.

Closes #55

🤖 Generated with Claude Code

[dfa1]

Closing: declining the ranged byte[] overloads. A byte[] lives on the JVM heap, so these always copy heap→native — they can't be zero-copy, only convenient. Widening the byte[] surface invites callers to reach for it on hot paths expecting efficiency (a footgun). Zero-copy stays the MemorySegment / direct-ByteBuffer path; the byte[] surface stays minimal.