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/shares/mod.rs

@@ -12,7 +12,11 @@ use std::io;
 use std::path::{Path, PathBuf};
 
 /// One shared file: its peer-facing virtual path and where it lives on disk.
+///
+/// Built by the scan, never by a host, so fields can be added without a
+/// breaking release.
 #[derive(Debug, Clone)]
+#[non_exhaustive]
 pub struct SharedFile {
     /// Backslash-separated path exposed to peers, e.g. `music\album\song.mp3`.
     pub virtual_path: String,