test(shared): cover null-valued entry in deserializePhantoms

Adds a null-valued entry under a valid square key (d3) to the
'keeps valid entries and drops invalid ones' fixture, proving the
typeof/null guard branch in deserializePhantoms is exercised.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/shared/test/phantoms.test.ts

@@ -32,6 +32,7 @@ describe('deserializePhantoms', () => {
       zz: { color: 'b', type: 'p' },  // invalid square
       a1: { color: 'x', type: 'p' },  // invalid colour
       b2: { color: 'b', type: 'z' },  // invalid type
+      d3: null,                       // valid square, null value
     });
     expect(deserializePhantoms(raw)).toEqual({ e5: { color: 'b', type: 'n' } });
   });