refactor(workspace): enforce layered crate boundaries

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

@@ -0,0 +1,45 @@
+use serde::{Deserialize, Serialize};
+
+use crate::{UsageEventError, UsageEventKind};
+
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+pub struct UsageEventEnvelope {
+    pub event_id: String,
+    pub request_id: String,
+    pub subject_id: String,
+    pub kind: UsageEventKind,
+    pub sequence: u64,
+}
+
+impl UsageEventEnvelope {
+    pub fn validate(&self) -> Result<(), UsageEventError> {
+        if self.event_id.trim().is_empty() {
+            return Err(UsageEventError::MissingEventId);
+        }
+        if self.request_id.trim().is_empty() {
+            return Err(UsageEventError::MissingRequestId);
+        }
+        if self.subject_id.trim().is_empty() {
+            return Err(UsageEventError::MissingSubjectId);
+        }
+        Ok(())
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::UsageEventEnvelope;
+    use crate::UsageEventKind;
+
+    #[test]
+    fn envelope_rejects_missing_identity() {
+        let event = UsageEventEnvelope {
+            event_id: String::new(),
+            request_id: "request".to_string(),
+            subject_id: "user".to_string(),
+            kind: UsageEventKind::Completed,
+            sequence: 1,
+        };
+        assert!(event.validate().is_err());
+    }
+}