feat: add learning objective targets

src/model/taxonomy.rs

@@ -9,6 +9,44 @@ use std::fmt;
 
 use serde::{Deserialize, Serialize};
 
+/// Which tier of the objective registry an entry or a reported row belongs to.
+///
+/// The two words come from the assessment literature and name two different
+/// jobs, not two sizes of the same thing. A **learning objective** is what a
+/// syllabus lists and what a report classifies as met: the unit a claim is made
+/// about. A **learning target** is the specific performance an item is written
+/// against and tagged to: what a student aims at in one class, and the evidence
+/// a claim about an objective rests on.
+///
+/// It lives here rather than beside the registry because every layer that
+/// reports needs it, and a `bool` named for one of the two tiers leaves the
+/// reader guessing which way round it points.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash, Serialize, Deserialize)]
+#[serde(rename_all = "snake_case")]
+pub enum Tier {
+    /// A learning objective: the tier a mastery claim is made about.
+    Objective,
+    /// A learning target: the tier items are tagged to, and evidence for the
+    /// objective above it.
+    Target,
+}
+
+impl Tier {
+    /// The snake_case token used in YAML and in flat storage.
+    pub fn as_str(self) -> &'static str {
+        match self {
+            Tier::Objective => "objective",
+            Tier::Target => "target",
+        }
+    }
+}
+
+impl fmt::Display for Tier {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str(self.as_str())
+    }
+}
+
 /// Cognitive demand, following the revised Bloom taxonomy.
 ///
 /// Serialized as the integers 1 through 5 so YAML reads `level: 3`. The derived