refactor(lib): keep the actors inside the crate and mark the growing enums

Every release since 14 has been a major, because the actor mailboxes,
the share index's struct and the CLI's command enum were all exhaustive
public API: adding a peer message, a scanned field or a subcommand read
as a breaking change to the library. The actors, dispatcher, peer
plumbing and download store are how the client is built, not what a host
programs against, so they move inside the crate; PeerMessage, SharedFile
and the CLI command enums are non-exhaustive; ConnectionType is
re-exported at the root for the one thing hosts used it for. A one-time
break so that the next additions are not.

soulseek-rs-lib/src/actor/mod.rs

@@ -52,7 +52,7 @@ impl<M: Send> ActorHandle<M> {
 }
 
 /// Internal actor message wrapper
-pub(crate) enum ActorMessage<M> {
+pub enum ActorMessage<M> {
     UserMessage(M),
     Stop,
 }

soulseek-rs-lib/src/actor/peer_actor.rs

@@ -21,7 +21,9 @@ use std::sync::mpsc::{Receiver, Sender};
 use std::sync::{Arc, RwLock};
 use std::time::{Duration, Instant};
 
+/// Marked non-exhaustive: each peer message the client learns adds a variant.
 #[derive(Debug, Clone)]
+#[non_exhaustive]
 pub enum PeerMessage {
     SendMessage(Message),
     FileSearchResult(SearchResult),