decoupled module system with exchange rates

generic_rust_todo.txt

@@ -0,0 +1,12 @@
+1.  - Your Rust module can define a column type XX, which you can add through the table definition.
+    - When a row is saved, the framework calls the registered modules’ row hooks.
+    - You implement your module’s hook and choose whether its logic runs for every table, only tables containing XX, or
+      another condition.
+
+    - If you want it to affect only the XX column, your hook selects and processes that column’s value from the supplied
+      row.
+
+    - The framework doesn’t automatically make those choices. You implement the filtering and column-specific behavior;
+      modules without a row-hook implementation default to doing nothing.
+
+2.