perf(utxo): read record identities directly

Hash-table probes and rehashes only need the txid prefix or txid, but the old accessors decoded output-count metadata on every call. Read those fixed identity bytes from the validated record header instead.

A 100,000-record v4 snapshot fixture ran 1.97x to 2.06x faster in forward and reverse 20-run comparisons; Callgrind instruction reads fell 54.3%.

Author: metaphorics

crates/utxo/src/record.rs

@@ -367,11 +367,15 @@ impl UtxoRecord {
     }
 
     pub(crate) fn key(&self) -> UtxoKey {
-        UtxoKey::from_txid(&self.txid())
+        let mut prefix = [0_u8; 8];
+        prefix.copy_from_slice(&self.buf.as_bytes()[..8]);
+        UtxoKey::from_prefix(prefix)
     }
 
     pub(crate) fn txid(&self) -> Hash256 {
-        self.header().txid
+        let mut txid = [0_u8; TXID_LEN];
+        txid.copy_from_slice(&self.buf.as_bytes()[..TXID_LEN]);
+        Hash256::from_le_bytes(&txid)
     }
 
     pub(crate) fn is_empty(&self) -> bool {