refactor(workspace): enforce layered crate boundaries

crates/aether-data/contracts/src/migration.rs

@@ -0,0 +1,7 @@
+//! Database lifecycle contracts shared by driver adapters and the data facade.
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct PendingMigrationInfo {
+    pub version: i64,
+    pub description: String,
+}