feat: robust docs

src/model/taxonomy.rs

@@ -1,5 +1,12 @@
 //! The pedagogical vocabulary: levels, cognitive processes, error types, and
 //! workflow states.
+//!
+//! These are enums rather than strings on purpose. A typo in
+//! `cognitive_process` should fail to parse, not silently create a new category
+//! that then splits your coverage report in two. Just as importantly, the
+//! relation between a level and the processes that belong to it is encoded here
+//! in one place, so "level 3, cognitive_process: recall" is a validation error
+//! rather than a label that quietly contradicts itself.
 
 use std::fmt;