CVE-2026-54542: nimiq-primitives: Out-of-bounds panic in KeyNibbles::Add from oversized child suffix in a deserialized proof

A malicious peer acting as a state-sync source can crash a syncing node with a crafted TrieChunk whose proof contains a TrieNodeChild whose suffix, when concatenated with the parent key via KeyNibbles::Add, exceeds the fixed 63-byte backing array. Add (primitives/src/key_nibbles.rs:332 / :341) indexes bytes[self.bytes_len()..self.bytes_len() + other.bytes_len()] with no combined-length check, causing an out-of-bounds slice panic (both the even- and odd-length branches).

KeyNibbles deserialization validates only the individual length <= 126, not the combined parent + suffix length. The panic occurs at put_chunk → child.key() → is_stump() → +, i.e. before proof.verify(), so no valid proof is required. As with the related child_index issue, exploitation requires being the victim’s sync peer during state sync, and the resulting crash is transient (the node restarts and re-syncs).

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