perf(runtime-vapor): use numeric keys for createIf branches (#14827)

packages/runtime-vapor/__tests__/if.spec.ts

@@ -279,7 +279,7 @@ describe('createIf', () => {
       ),
     ).render()
 
-    expect(branch.$key).toBe('00')
+    expect(branch.$key).toBe(0)
   })
 
   // vapor custom directives have no lifecycle hooks.

packages/runtime-vapor/src/apiCreateIf.ts

@@ -62,7 +62,7 @@ export function createIf(
       }
       ;(frag as DynamicFragment).update(
         ok ? b1 : b2,
-        keyed ? `${index}${ok ? 0 : 1}` : undefined,
+        keyed ? index * 2 + (ok ? 0 : 1) : undefined,
       )
     })
   }