Add custom item models for various shops using paper as the base item

data/new_music/function/custom_items/give.mcfunction

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+# Generic helper for giving custom items with a custom model.
+# Example usage:
+# function new_music:custom_items/give {item:"minecraft:paper", model:"city_under_siege6"}
+give @s $(item)[minecraft:custom_model_data={strings:["$(model)"]}]

data/new_music/function/custom_items/register.mcfunction

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+# Generic registration helper for custom item models.
+# This keeps the item namespace separate from the display model name.
+# Usage: function new_music:custom_items/register {item:"minecraft:paper", model:"city_under_siege6"}
+
+# The resource pack handles the visual model via the shared item selector.
+# This function exists as a general entry point for future custom items.