Trie: reject ROOT-keyed items in put_chunk

[jsdanielh]

What's in this pull request?

MerkleRadixTrie::put_chunk processes TrieChunk data from state-sync peers. It validates sorting, key range, and the Merkle proof but does not reject items whose key is KeyNibbles::ROOT (the empty-nibbles root key). When such an item reaches put_raw, the equal-key branch calls cur_node.put_value(value).unwrap() on the root node.

A malicious sync peer crafts a ResponseChunk::Chunk whose items[0].key == KeyNibbles::ROOT. ROOT is the lexicographically smallest key, so the sort check passes. The proof verifies because it is over the last item's Merkle path, not every item. The crash fires on the first chunk the node commits.

Fix

1. Reject any item whose key is empty, inside the existing if !chunk.items.is_empty() { … } block, using the existing InvalidChunk error variant. items[0] is sufficient because the existing sort check guarantees strictly increasing keys and ROOT is the minimum.

Impact

• Consensus / wire format / public API: none. InvalidChunk is already returned for other chunk validation failures; commit_chunks already aborts the txn, logs a warn, and triggers reset_chunk_request_chain() in the live-sync layer. Honest peers never construct ROOT-keyed items.