feat: add invariant-safe workspace state

crates/ferese-layout/src/lib.rs

@@ -2,6 +2,10 @@ 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;
 
@@ -121,6 +125,10 @@ impl LayoutTree {
         self.windows.contains_key(&window)
     }
 
+    pub fn window_ids(&self) -> impl Iterator<Item = WindowId> + '_ {
+        self.windows.keys().copied()
+    }
+
     pub fn insert(
         &mut self,
         window: WindowId,