feat: core domain types and Filter predicate

tests/model_tests.rs

@@ -0,0 +1,20 @@
+use alpha_id::model::{Filter, Tags, Mode};
+
+#[test]
+fn filter_billable_only_rejects_non_primary() {
+    let tags = Tags { billable: false, valid: true, chapter: "I".into(), exotic: false };
+    let f = Filter { billable_only: true, valid_only: false, chapters: None, exclude_exotic: false };
+    assert!(!f.accepts(&tags));
+}
+
+#[test]
+fn filter_default_accepts_everything() {
+    let tags = Tags { billable: false, valid: false, chapter: "II".into(), exotic: true };
+    assert!(Filter::default().accepts(&tags));
+}
+
+#[test]
+fn mode_parses_from_str() {
+    assert_eq!("a".parse::<Mode>().unwrap(), Mode::A);
+    assert_eq!("c".parse::<Mode>().unwrap(), Mode::C);
+}