Lz4Decompressor.decompressFrame

Decompress an LZ4 frame contained in _inBuffer.

The implementation follows the LZ4 frame format:

• Verifies and skips the 4-byte magic.
• Parses FLG/BD bytes and skips optional content size and dictionary ID fields.
• Iterates over blocks, decoding either raw (uncompressed) blocks or compressed blocks via lz4DecompressBlock.
• Skips optional per-block and content checksums (not yet validated).

Only independent-block frames are supported; dependent-block frames result in ErrorCode.UNSUPPORTED_FORMAT.