`Encoder` class lacks a `code` property exposing the prefix byte

The Encoder class stores the prefix code internally as self._codec.code but doesn't expose it as a public property. Users who need the prefix byte must access the private _codec attribute or call get_encoding_info().

Problem

enc = Encoder("base16")
enc.encoding   # "base16" ✅ — name is public
enc.code       # ❌ AttributeError — prefix byte not exposed
enc._codec.code  # b"f" — works but accesses private attribute

Go's Encoder exposes the encoding code:

func (p Encoder) Encoding() Encoding {
    return p.enc  // Returns the Encoding constant (e.g., Base16 = 'f')
}

Proposed Solution

Add a code property to the Encoder class:

class Encoder:
    @property
    def code(self) -> bytes:
        """The multibase prefix byte for this encoding."""
        return self._codec.code

Add tests and update __init__.py exports if needed.

Related

• Go equivalent: Encoder.Encoding() method in go-multibase encoder.go

Start by locating the Python Encoder class and its existing tests, then inspect how the internal _codec.code value is exposed. Add the public code property described in the issue, update init.py exports only if needed, and add tests confirming the returned prefix byte for an encoding such as base16.