Deterministic CBOR encoder/decoder.

Implements the subset of RFC 8949 needed by Macula records: unsigned ints, byte strings, text strings, arrays, maps, and null.

Encoding follows RFC 8949 §4.2.1 (deterministic):

• Smallest length encoding.
• Definite lengths only (no indefinite items).
• Map keys sorted by bytewise lexicographic order of their deterministic encoding.

Internal value representation:

• non_neg_integer() — uint (major 0)
• binary() — byte string (major 2)
• {text, binary()} — UTF-8 text string (major 3)
• atom() — UTF-8 text string (major 3) via atom_to_binary/1. Accepted for round-trip robustness: the frame decoder atomizes binary keys via binary_to_existing_atom/1, so a record decoded from the wire and re-encoded for sig verify carries atom keys inside the payload sub-map. Encoding atoms as text reproduces the original wire bytes byte-for-byte (atom names round-trip exactly through utf8). null has a dedicated clause and is NOT routed here.
• [value()] — array (major 4)
• #{value() => value()} — map (major 5)
• null — simple null (major 7, value 22)