verify_compact_trie_proof: avoid per-iteration partial_key allocation

[x3c41a]

Follow-up from #3263 (discussion).

compact_proof_compute_merkle in lib/src/trie/proof_decode.rs allocates a Vec<Nibble> per descend iteration via decoded.partial_key.clone().collect(). Avoiding it requires adding a lifetime to CompactFrame so partial_key can borrow from node_bytes.