قَدِمْتَ قُدُومًا حَسَنًا: an added explanatory sentence with explicit verb and maṣdar.
خَيْرَ مَقْدَمٍ: the arrival context allows the verb to remain implicit.
Ellipsis is licensed by understood context; it is not a licence to delete a verb arbitrarily.
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The Accusatives: Al-Mafʿūl al-Muṭlaq
قَدِمْتَ قُدُومًا حَسَنًا: an added explanatory sentence with explicit verb and maṣdar.
خَيْرَ مَقْدَمٍ: the arrival context allows the verb to remain implicit.
Ellipsis is licensed by understood context; it is not a licence to delete a verb arbitrarily.