refactor(scale_degree): ♻️ extract romanNumeral getter

lib/src/scale/scale_degree.dart

@@ -149,6 +149,25 @@ class ScaleDegree implements Comparable<ScaleDegree> {
         semitonesDelta: semitonesDelta,
       );
 
+  /// Returns the roman numeral of this [ScaleDegree] based on [ordinal].
+  ///
+  /// Example:
+  /// ```dart
+  /// ScaleDegree.i.romanNumeral == 'I'
+  /// ScaleDegree.vii.romanNumeral == 'VII'
+  /// ScaleDegree.neapolitanSixth.romanNumeral == 'II'
+  /// ```
+  String get romanNumeral => switch (ordinal) {
+        1 => 'I',
+        2 => 'II',
+        3 => 'III',
+        4 => 'IV',
+        5 => 'V',
+        6 => 'VI',
+        7 => 'VII',
+        _ => '$ordinal',
+      };
+
   @override
   String toString({
     ScaleDegreeNotation system = ScaleDegreeNotation.standard,
@@ -203,33 +222,16 @@ final class StandardScaleDegreeNotation extends ScaleDegreeNotation {
 
   @override
   String scaleDegree(ScaleDegree scaleDegree) {
-    final buffer = StringBuffer();
-    if (scaleDegree.semitonesDelta != 0) {
-      buffer.write(Accidental(scaleDegree.semitonesDelta).symbol);
-    }
-    final romanNumeral = switch (scaleDegree.ordinal) {
-      1 => 'I',
-      2 => 'II',
-      3 => 'III',
-      4 => 'IV',
-      5 => 'V',
-      6 => 'VI',
-      7 => 'VII',
-      _ => '',
-    };
-
-    if (scaleDegree.quality != null && scaleDegree.quality!.semitones <= 0) {
-      buffer.write(romanNumeral.toLowerCase());
-    } else {
-      buffer.write(romanNumeral);
-    }
-
-    switch (scaleDegree.inversion) {
-      case 1:
-        buffer.write('6');
-      case 2:
-        buffer.write('64');
-    }
+    final buffer = StringBuffer()
+      ..writeAll([
+        if (scaleDegree.semitonesDelta != 0)
+          Accidental(scaleDegree.semitonesDelta).symbol,
+        if (scaleDegree.quality != null && scaleDegree.quality!.semitones <= 0)
+          scaleDegree.romanNumeral.toLowerCase()
+        else
+          scaleDegree.romanNumeral,
+        switch (scaleDegree.inversion) { 1 => '6', 2 => '64', _ => '' },
+      ]);
 
     return buffer.toString();
   }

lib/src/scale/scale_pattern.dart

@@ -337,18 +337,19 @@ final class ScalePattern {
       return ChordPattern.fromQuality(scaleDegree.quality!);
     }
 
-    Interval step(int ordinal) =>
-        intervalSteps[(ordinal - 1) % intervalSteps.length];
-
-    Interval addNextStep(int ordinal) => step(ordinal) + step(ordinal + 1);
-
-    // Calculate the diatonic pattern from this Scale's `intervalSteps`.
+    // Deduce the diatonic `ChordPattern` from this `Scale.intervalSteps`.
     return ChordPattern.fromIntervalSteps([
-      addNextStep(scaleDegree.ordinal),
-      addNextStep(scaleDegree.ordinal + 2),
+      _addNextStepTo(scaleDegree.ordinal),
+      _addNextStepTo(scaleDegree.ordinal + 2),
     ]);
   }
 
+  Interval _stepFrom(int ordinal) =>
+      intervalSteps[(ordinal - 1) % intervalSteps.length];
+
+  Interval _addNextStepTo(int ordinal) =>
+      _stepFrom(ordinal) + _stepFrom(ordinal + 1);
+
   /// Whether this [Scale] is enharmonically equivalent to [other].
   ///
   /// Example:

test/src/scale/scale_degree_test.dart

@@ -110,6 +110,23 @@ void main() {
       });
     });
 
+    group('.romanNumeral', () {
+      test('returns the roman numeral of this ScaleDegree', () {
+        expect(ScaleDegree.i.romanNumeral, 'I');
+        expect(ScaleDegree.ii.minor.romanNumeral, 'II');
+        expect(ScaleDegree.iii.minor.inverted.romanNumeral, 'III');
+        expect(ScaleDegree.iv.major.lowered.romanNumeral, 'IV');
+        expect(ScaleDegree.v.minor.raised.romanNumeral, 'V');
+        expect(ScaleDegree.vi.romanNumeral, 'VI');
+        expect(ScaleDegree.vii.romanNumeral, 'VII');
+      });
+
+      test('returns the ordinal number if higher than 7', () {
+        expect(const ScaleDegree(8).romanNumeral, '8');
+        expect(const ScaleDegree(20).romanNumeral, '20');
+      });
+    });
+
     group('.toString()', () {
       test('returns the string representation of this ScaleDegree', () {
         expect(ScaleDegree.i.toString(), 'I');