nimiq-primitives: Panic DoS in trie chunk processing via ROOT-keyed item

### Impact A remote, unauthenticated denial-of-service vulnerability in `MerkleRadixTrie::put_chunk` allows any state-sync peer to crash any node performing state synchronization (freshly joining nodes and recovering nodes). A malicious peer can respond to a `RequestChunk` with a `ResponseChunk::Chunk` whose first `TrieItem.key` is the empty (ROOT) key. The chunk passes sorting, range, and Merkle-proof validation, but when `put_raw` tries to store a value at the root node, it calls `TrieNode::put_value(...).unwrap()`, which returns `Err(RootCantHaveValue)` and panics, aborting the node process. The panic fires on the first malicious chunk the victim commits; no rate limit or authentication gate caps the attack. Impacted: any node running state sync against untrusted peers — this includes fresh nodes performing initial download and existing nodes recovering from data loss. Honest nodes never construct ROOT-keyed items, so non-syncing operation is unaffected. ### Patches See [PR](https://github.com/nimiq/core-rs-albatross/pull/3762). ### Workarounds There is no safe in-process workaround: any peer serving state-sync data can trigger the crash and the code path is not guarded by a feature flag. ### Resources - Fix commit: (link to the merged PR commit, once merged) - Affected code: [`primitives/trie/src/trie.rs`](https://github.com/nimiq/core-rs-albatross/blob/albatross/primitives/trie/src/trie.rs) — `put_chunk` (around line 819) and `put_raw` (around line 351)