Core: Fix FixedByteBufferWriter to use writeFixed() for Avro fixed[N] fields

[DuanRuixiao]

Problem

FixedByteBufferWriter.write() was calling encoder.writeBytes(), which prepends a zigzag-encoded length prefix before the payload. For Avro fixed[N] fields this is wrong — the reader expects exactly N bytes with no prefix. The stray length byte spills into the next field in the record and corrupts it.

For example, writing a FIXED(3) partition value [AB CD EF] produced [06 AB CD EF] on disk (where 06 is zigzag(3)). The reader then consumed [06 AB CD] as the partition value, and the leftover [EF] corrupted the subsequent record_count field, decoding it as -568 instead of the actual row count.

The symmetric read path in InternalReader had the same issue: case FIXED: fell through to case BYTES: and used byteBuffers() (decoder.readBytes()), which reads a length-prefixed byte sequence instead of a fixed-size one.

Fix

• ValueWriters: FixedByteBufferWriter.write() now extracts the bytes into a byte[] and calls encoder.writeFixed() (exact N bytes, no prefix), matching the existing FixedWriter for byte[].
• ValueReaders: Add FixedByteBufferReader that calls decoder.readFixed(bytes, 0, length) / decoder.skipFixed(length). Expose it via ValueReaders.fixedBuffers(int length).
• InternalReader: case FIXED: now returns ValueReaders.fixedBuffers(primitive.getFixedSize()) instead of falling through to the bytes reader.

Tests

TestFixedByteBufferWriter adds two regression tests:

• testFixedWriterProducesExactBytes — directly verifies the encoder emits exactly N bytes with no length prefix.
• testManifestRoundTripWithFixedPartition — writes and reads back a manifest with a FIXED(3) identity-partition column; asserts both the partition value and record_count survive the round-trip uncorrupted.

[huaxingao]

Thanks for tracking this down, @DuanRuixiao. The root cause is right: FixedByteBufferWriter uses writeBytes() for a fixed[N] field instead of writeFixed().

A couple of things:

1. Iceberg's own reader isn't affected. InternalReader's case FIXED falls through to readBytes(), which matches the writer's writeBytes(), so Iceberg reads its own manifests fine (a ManifestFiles round-trip returns record_count = 4). The corruption only shows up in spec-compliant external readers that do readFixed.

2. The fix flips the on-disk format both ways.

writer \ reader	old reader	new reader
old writer (existing manifests)	✅ 4	❌ corrupted (-568 in the example)
new writer (new manifests)	❌ error	✅ 4

So the new reader misreads every existing manifest with a fixed field, and new manifests can't be read by older Iceberg. I think we need an approach that doesn't break existing data.