Assignment shorhands to private stateful field aren't proxified#15832

### Describe the bug ```js // input #c = $state(); ... this.#c ??= {val: 0} // observed output $.set(this.#c, this.#c.v ?? { val: 0 }); // expected output, I guess $.set(this.#c, this.#c.v ?? { val: 0 }, true); ``` ### Reproduction [REPL](https://svelte.dev/playground/hello-world?version=5.28.2#H4sIAAAAAAAAA32RTW6DMBCFrzJyukiUCpotBaKqm94hZOGYSWvFNcgeSCOLu3fMj9JKVRd4zJvvDc8mCCs_UWTiDY1p4No4U8Maa01Yb8SjOGuDXmSHIOjWRi4KrM-ul7ZNfI-GonaSHv_SVWMJLfEYkXvldEtlZStSRnoPr03HXQchShWtFBTw4EkSrjfPk8Z-T65T1Lj1ZgErog_tE-b3-wJCL00GT8PUm4u2KvIwgwkzsC1gB3P_HQlYGxmH1Dn7Cx2puHAxjKo5aQEWr0vuOWTsX_B2z76Lep7ej2vDKgL8jHPzU0fUWGisMlpdisApinL5RjJGH0oueTqR5T8uHrrdjrSL-5-WaGrLPoNDWGbz2YZjnrbcDumUh38S4ReJjK8ZhyO_SW2u2tYiO0vjcfgGGIF29icCAAA=) ### Logs ```shell ``` ### System Info ```shell 5.28.2 ``` ### Severity annoyance