session: extract Session from memory_search to src/session.rs

Generic session state (session_id, seen set, state directory) doesn't
belong in the memory search module. Now at crate root, re-exported
from memory_search for backwards compatibility.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/lib.rs

@@ -16,6 +16,9 @@ pub mod subconscious;
 // Unified configuration
 pub mod config;
 
+// Session state
+pub mod session;
+
 // Shared utilities
 pub mod util;