Fix protocol version/header parsing

Basically, the replay parser entirely ignores important fields like `headerSize`, doesn't check magic, network version and so on and so on.

This is a problem for engine https://github.com/beyond-all-reason/RecoilEngine/pull/2714 and very negatively contributes to the ecosystem blocking further changes.

Start by locating the replay parser and inspect how it reads the demo header. Trace handling of headerSize and validation of the magic and network version, then use the engine issue and linked RecoilEngine pull request as context for expected protocol behavior. Done means these header fields are parsed and invalid values are checked rather than ignored.

Written by the indexing model from the issue text.