fix(daemon): correct the published contract and harden the control socket

A review pass over the diff found seventeen confirmed defects. The worst was
the contract itself: every method's parameters were published as a single
descriptor named `params`, so a client generated from openrpc.json sent
{"params":{"params":{…}}} and every call failed -32602. Parameters are now
published per field, with a test asserting the published names are the keys
the daemon actually deserializes.

Security and lifecycle:
- download.start no longer lets a caller name a directory, which was an
  arbitrary write on the daemon's host.
- The Unix socket is bound under a temporary name and renamed into place; it
  was briefly connectable by any local user between bind and chmod, and on
  that transport connecting is authenticating.
- Pre-auth reads are capped at 1 MiB with a 30s handshake timeout, and
  connections are capped, so an unauthenticated peer cannot exhaust memory or
  threads.
- A read error no longer returns past the hub unsubscribe, which leaked a
  subscriber and two threads per occurrence.
- A client whose event queue fills is now actually disconnected. It used to be
  dropped from the fan-out and left connected, working on but blind to every
  event after — the documentation already promised otherwise.
- A client whose daemon vanishes mid-transfer is told at once instead of
  waiting out its full timeout and reporting a misleading "gave up".

Contract accuracy: errors carry an explicit null id (a missing one reads as a
notification and gets dropped), a second auth answers with the published
shape, and nullable fields are sent rather than omitted so a generated client
sees one shape.

Also splits proto.rs and server.rs, whose concerns had diverged, and applies
the review's simplifications.

soulseek-rs/src/commands/transfer.rs

@@ -1,6 +1,7 @@
 //! Finding files and fetching them: `search`, `download`, and `get`.
 
 use super::{Ctx, Session, social};
+use crate::api::SessionApi;
 use crate::cli::{
     DownloadArgs, FilterArgs, GetArgs, Pick, SearchArgs, SortKey,
 };
@@ -8,7 +9,6 @@ use crate::output::{
     CliError, CliResult, DownloadRecord, Exit, Out, SearchRecord,
 };
 use soulseek_rs::utils::path::expand_tilde;
-use crate::api::SessionApi;
 use soulseek_rs::{DownloadStatus, SearchResult};
 use std::collections::{HashMap, VecDeque};
 use std::io::BufRead;