Rework protocol IO, add Buffer abstraction & fix socket write issues

[ItsDrike]

The previous protocol implementation mixed multiple responsibilities in Connection. It acted both as a network transport and as an in-memory buffer for packet construction/parsing. This made the code confusing, hard to reason about, and resulted in duplicated serialization logic.

This change restructures the protocol IO layer and cleans up the design:

• Extract binary read/write primitives into _protocol.io.base_io (BaseSync/AsyncReader and BaseSync/AsyncWriter).
• Introduce a dedicated Buffer type for in-memory packet construction and decoding instead of abusing connection objects as buffers.
• Move socket implementations into _protocol.io.connection.
• Update protocol clients to use the new IO abstractions and the StructFormat helpers for typed serialization.

Besides the structural cleanup, this also fixes several important issues in the previous implementation:

• The synchronous TCP connection used socket.send(data), which does not guarantee that all bytes are sent. The new implementation correctly uses socket.sendall().

• The asynchronous TCP write method was synchronous. It was just calling writer.write(data) but never awaited drain(), meaning the actual network write could happen at an arbitrary later time. In practice this rarely broke because a read always followed the write, which implicitly allowed the event loop to flush the buffer, but the behavior was still incorrect and fragile. The new async connection now uses async write method and awaits writer.drain() to ensure proper write semantics.

Inspiration

A lot of the new design takes direct inspiration from how mcproto handles protocol interactions.

[PerchunPak]

LGTM