Introduce a managed custom item API with mine-block callbacks and cancellation semantics, plus native runtime support for registering pickaxe items.
Key changes:
- add WeaveLoader.API Item base/PickaxeItem and dispatcher plumbing
- register managed item instances in ItemRegistry
- add native export for pickaxe registration and wire through GameObjectFactory
- resolve/hook item mineBlock paths (ItemInstance/Item/DiggerItem) and dispatch to managed host
- expose managed OnItemMineBlock entry in WeaveLoader.Core and DotNetHost
- add Ruby Pickaxe example item + placeholder texture
- keep logger usable even before managed handler setup via native fallback
