GetStorage and DeleteStorage used GetBinaryTreeKey to compute the tree
key, while UpdateStorage used GetBinaryTreeKeyStorageSlot. The latter
applies storage slot remapping (header offset for slots <64, main
storage prefix for the rest), so reads and deletes were targeting
different tree locations than writes.
Replace GetBinaryTreeKey with GetBinaryTreeKeyStorageSlot in both
GetStorage and DeleteStorage to match UpdateStorage. Add a regression
test that verifies the write→read→delete→read round-trip for main
storage slots.
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