A minimal truecolor PNG encoder over Chez bytevectors — CRC-32 / Adler-32
framing with DEFLATE "stored" blocks, so there's no compressor to carry. Restores
the jolt.png built-in (image/put!/write) the old host provided; def-var!'d into
the jolt.png namespace and loaded in the CLI before the loader's baked-namespace
snapshot, so (require '[jolt.png]) resolves with no source file. Output verified
to decode as a valid PNG (signature, IHDR, CRC-correct chunks, inflatable IDAT).
