The encoder needs to produce a DEFLATE stream, and the simplest valid one wraps raw bytes in a stored block. Today you emit that block, guaranteed to inflate back through the decoder you already built.
Wrap raw bytes in a single final stored (type 0) DEFLATE block.
The encoder’s easiest path to a valid DEFLATE stream is a stored block: no compression, just framing. Emit one header byte 0x01 - BFINAL set (this is the only, final block) and BTYPE 00 (stored) - then the 2-byte little-endian LEN, its complement NLEN, and the raw bytes. For Hi that is 01 02 00 FD FF 48 69, and it is a closed loop: Inflate from chapter four turns it straight back into Hi.
This is honest and complete on its own - an encoder that only ever emits stored blocks produces perfectly valid PNGs, just uncompressed ones. It also gives you an end-to-end round trip today: pixels to scanlines to a stored block that your decoder inflates back. For real files longer than 65535 bytes you would split into multiple stored blocks (each LEN maxes at that), but the mechanism is identical. Actual compression - a fixed-Huffman block that shrinks the data - comes shortly; the stored block is the reliable floor beneath it.
// header byte 0x01 = BFINAL(1) + BTYPE(00). Then LEN and NLEN as 2-byte// little-endian (NLEN = ^LEN), then the raw bytes verbatim.func storedBlock(data []byte) []byte { }