Content-coding codec behaviour and registry.

A codec turns response bytes into a single content-coding (the token sent in Content-Encoding, e.g. gzip). livery_compress negotiates the client's Accept-Encoding against the registered codecs and applies the chosen one to {full, _} and {chunked, _} bodies.

The built-in codecs livery_codec_gzip and livery_codec_deflate are always available (over OTP zlib, no dependency). A separate app (a future livery_brotli or livery_zstd) adds its coding by calling register/1 at its own application start; it then participates in negotiation without any change to livery core.

Callbacks

• name/0 — the Content-Encoding token (lowercase binary).
• compress/1 — one-shot compression of a whole body ({full, _}).
• stream_init/0 — open a streaming context.
• stream_update/2 — feed a chunk and FLUSH, returning the bytes emittable now (so each producer chunk reaches the client promptly).
• stream_finish/1 — return the trailing bytes that finalize the stream. Does not release the context.
• stream_close/1 — release the context; always called, including on the error path.