Give the message log to Messages as its own fields

Not a pure move: state moves. Pardes's three message-log fields
(`messages: [message_log]LoggedMessage`, `messages_head`, `messages_len`)
become the fields of Messages.zig itself (`log`, `head`, `len`), and
Pardes embeds one as `messages: Messages = .{}`. messageLog reads only the
ring, so it now takes `m: *const Messages` and is called
`p.messages.messageLog(i)` (Output's +Messages view and the log test);
logMessage still needs the panes and the 9P event log, so it keeps
`p: *Pardes` and writes `p.messages.log/head/len`. No behaviour change.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

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