refactor(workspace): enforce layered crate boundaries

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

@@ -0,0 +1,20 @@
+use serde::{Deserialize, Serialize};
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
+pub enum UsageEventKind {
+    Started,
+    Streaming,
+    Completed,
+    Failed,
+    Cancelled,
+}
+
+#[derive(Debug, thiserror::Error, PartialEq, Eq)]
+pub enum UsageEventError {
+    #[error("usage event id cannot be empty")]
+    MissingEventId,
+    #[error("usage request id cannot be empty")]
+    MissingRequestId,
+    #[error("usage subject id cannot be empty")]
+    MissingSubjectId,
+}