CVE-2026-54541: nimiq-primitives: Panic in TrieProof::verify via child_index unwrap on equal-length keys

A malicious peer acting as a state-sync source can crash a syncing node by sending a crafted TrieChunk whose proof contains two TrieProofNodes with identical keys. TrieProof::verify() calls TrieProofNode::child_index() (primitives/src/trie/trie_proof_node.rs:94), which unconditionally unwraps KeyNibbles::get(self.key.len()). Because is_prefix_of returns true for two equal keys, execution reaches get(len), which returns None, and the unwrap() panics.

The panic is reached from untrusted network input (ResponseChunk → commit_chunks → put_chunk → proof.verify()) before any cryptographic proof verification, so the attacker does not need to produce a valid proof. Exploitation requires the attacker to be selected as the victim’s sync peer while the victim is performing state sync, and the resulting crash is transient (the node restarts and re-syncs).

Affected: core-rs-albatross <= 1.5.1 (nimiq-primitives).