fix VerseRef.verseNum setter mis-port

CLAUDE.md

@@ -138,9 +138,6 @@ porting a test — several `VerseRefTests` cases can't pass yet, and it's not ob
   needs `Versification.getLastBook/getLastChapter/getLastVerse`, which needs the `.vrs`
   versification data ported — see the stalled `improve-verseref` branch.
 - **`VerseRef.isExcluded`** is hardcoded `return false` with a `TODO`.
-- **`VerseRef.set verseNum`** is a mis-port still carrying a `ToDo`: it assigns the backing field
-  but omits the C# negative guard _and_ the `verse = null` that clears a range string. So setting
-  `verseNum` on `'LUK 3:4b-5a'` leaves a stale `'4b-5a'` in the `verse` getter.
 - **`BBBCCCVVVS`** is declared but not implemented.
 - **The numeric constructor bypasses the setters**, assigning `_bookNum`/`_chapterNum`/`_verseNum`
   directly. So the C# `Invalid` test's constructor-throws cases (e.g. `new VerseRef(2, -42, 1)`)

src/verse-ref.test.ts

@@ -223,6 +223,40 @@ describe('VerseRef', () => {
         vref.chapterNum = 0;
         expect(vref.chapterNum).toEqual(0);
       });
+
+      it('should throw when verseNum is negative', () => {
+        const vref = new VerseRef('LUK 3:4', ScrVers.English);
+        expect(() => {
+          vref.verseNum = -1;
+        }).toThrow(VerseRefException);
+      });
+
+      it('should not throw when verseNum is zero', () => {
+        const vref = new VerseRef('LUK 3:4', ScrVers.English);
+        vref.verseNum = 0;
+        expect(vref.verseNum).toEqual(0);
+      });
+
+      it('should clear a verse range when verseNum is set', () => {
+        const vref = new VerseRef('LUK', '3', '4b-5a', ScrVers.English);
+        expect(vref.verse).toEqual('4b-5a');
+        expect(vref.hasMultiple).toBe(true);
+
+        vref.verseNum = 9;
+
+        expect(vref.verseNum).toEqual(9);
+        expect(vref.verse).toEqual('9');
+        expect(vref.hasMultiple).toBe(false);
+      });
+
+      it('should clear a verse segment when verseNum is set', () => {
+        const vref = new VerseRef('LUK', '3', '4b', ScrVers.English);
+        expect(vref.verse).toEqual('4b');
+
+        vref.verseNum = 9;
+
+        expect(vref.verse).toEqual('9');
+      });
     });
 
     describe('String', () => {

src/verse-ref.ts

@@ -353,8 +353,11 @@ export class VerseRef {
     return this._verseNum;
   }
   set verseNum(value: number) {
-    // ToDo: replace or remove this placeholder
+    if (value < 0) {
+      throw new VerseRefException('VerseNum can not be negative');
+    }
     this._verseNum = value;
+    this._verse = undefined;
   }
 
   /**