كِتَابُ زَيْدٍ — Zayd’s book. The noun kitāb becomes definite through iḍāfa to the proper name Zayd.
نَظَرْتُ إِلَى كِتَابِ زَيْدٍ — I looked at Zayd’s book.
كِتَابِ: genitive after ilā, with kasra; as muḍāf, it has no tanwīn.
زَيْدٍ: muḍāf ilayh, genitive with kasra and tanwīn.
No tanwīn on a construct-state noun is not evidence that it is a diptote.
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Definiteness and Proper-Name Status in Diptote Rules
كِتَابُ زَيْدٍ — Zayd’s book. The noun kitāb becomes definite through iḍāfa to the proper name Zayd.
نَظَرْتُ إِلَى كِتَابِ زَيْدٍ — I looked at Zayd’s book.
كِتَابِ: genitive after ilā, with kasra; as muḍāf, it has no tanwīn.
زَيْدٍ: muḍāf ilayh, genitive with kasra and tanwīn.
No tanwīn on a construct-state noun is not evidence that it is a diptote.