feat(shares): scan a shared directory into an in-memory index

Add a std-only shares module: Shares::scan walks the shared directory (skipping
symlinks and unreadable entries) into a read-only snapshot keyed by peer-facing
virtual path (root name + backslash-separated relative path). Exposes
search (case-insensitive multi-term AND), get (O(1) by virtual path), and
file/folder counts for the login handshake. Unit-tested against a temp tree.

soulseek-rs-lib/src/lib.rs

@@ -6,6 +6,7 @@ pub mod download_store;
 pub mod error;
 pub mod message;
 pub mod peer;
+pub mod shares;
 pub mod types;
 #[macro_use]
 pub mod utils;