refactor(workspace): enforce layered crate boundaries

crates/aether-task/core/src/fencing.rs

@@ -0,0 +1,16 @@
+#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct FencingToken(u64);
+
+impl FencingToken {
+    pub const fn new(value: u64) -> Option<Self> {
+        if value == 0 {
+            None
+        } else {
+            Some(Self(value))
+        }
+    }
+
+    pub const fn get(self) -> u64 {
+        self.0
+    }
+}