refactor(workspace): enforce layered crate boundaries

crates/aether-usage/core/src/settlement.rs

@@ -0,0 +1,12 @@
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct SettlementInput {
+    pub request_id: String,
+    pub event_sequence: u64,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum SettlementDisposition {
+    Apply,
+    AlreadyApplied,
+    RejectOutOfOrder,
+}