refactor(workspace): enforce layered crate boundaries

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

@@ -0,0 +1,22 @@
+pub fn partition_for_subject(subject_id: &str, partitions: u16) -> u16 {
+    if partitions == 0 {
+        return 0;
+    }
+    let hash = subject_id.bytes().fold(0u32, |acc, byte| {
+        acc.wrapping_mul(16777619).wrapping_add(u32::from(byte))
+    });
+    (hash % u32::from(partitions)) as u16
+}
+
+#[cfg(test)]
+mod tests {
+    use super::partition_for_subject;
+
+    #[test]
+    fn subject_partition_is_stable() {
+        assert_eq!(
+            partition_for_subject("user-1", 16),
+            partition_for_subject("user-1", 16)
+        );
+    }
+}