refactor: move workspace policy into core

crates/ferese-layout/src/lib.rs

@@ -2,10 +2,6 @@ use std::collections::{HashMap, HashSet};
 use std::error::Error;
 use std::fmt;
 
-mod workspace;
-
-pub use workspace::{Workspace, WorkspaceError, WorkspaceId, WorkspaceSet};
-
 const MIN_SPLIT_RATIO: f64 = 0.05;
 const MAX_SPLIT_RATIO: f64 = 0.95;