perf(binary): unpack packed values through a byte-pair table

wacore/binary/benches/binary_benchmark.rs

@@ -22,6 +22,44 @@ fn create_small_node() -> Node {
         .build()
 }
 
+/// The shape the wire is mostly made of: a short ack whose `to` is a JID and
+/// whose `id` is hex, so decoding it goes through `read_jid_pair` and
+/// `read_packed`. `create_small_node` has neither, which left the two paths
+/// that dominate a small-stanza profile uncovered.
+fn create_ack_node() -> Node {
+    NodeBuilder::new("ack")
+        .attr("to", "5511999990000@s.whatsapp.net")
+        .attr("id", "3EB0A1B2C3D4E5F60718")
+        .attr("class", "message")
+        .build()
+}
+
+/// A device fanout, where the same two paths repeat per child.
+fn create_fanout_node() -> Node {
+    let devices: Vec<Node> = (0..8)
+        .map(|i| {
+            NodeBuilder::new("to")
+                .attr("jid", format!("5511999990000:{i}@s.whatsapp.net"))
+                .children(vec![
+                    NodeBuilder::new("enc")
+                        .attr("v", "2")
+                        .attr("type", "msg")
+                        .bytes(vec![0xAB; 128])
+                        .build(),
+                ])
+                .build()
+        })
+        .collect();
+    NodeBuilder::new("message")
+        .attr("to", "5511999990000@g.us")
+        .attr("id", "3EB0A1B2C3D4E5F60718")
+        .attr("type", "text")
+        .children(vec![
+            NodeBuilder::new("participants").children(devices).build(),
+        ])
+        .build()
+}
+
 fn create_large_node() -> Node {
     NodeBuilder::new("iq")
         .attr("to", "server@s.whatsapp.net")
@@ -243,6 +281,32 @@ fn bench_unmarshal_small(bencher: divan::Bencher) {
         });
 }
 
+fn setup_ack_marshaled() -> Vec<u8> {
+    marshal(&create_ack_node()).unwrap()
+}
+
+fn setup_fanout_marshaled() -> Vec<u8> {
+    marshal(&create_fanout_node()).unwrap()
+}
+
+#[divan::bench]
+fn bench_unmarshal_ack(bencher: divan::Bencher) {
+    bencher
+        .with_inputs(setup_ack_marshaled)
+        .bench_refs(|marshaled| {
+            black_box(unmarshal_ref(black_box(&marshaled[1..])).unwrap());
+        });
+}
+
+#[divan::bench]
+fn bench_unmarshal_fanout(bencher: divan::Bencher) {
+    bencher
+        .with_inputs(setup_fanout_marshaled)
+        .bench_refs(|marshaled| {
+            black_box(unmarshal_ref(black_box(&marshaled[1..])).unwrap());
+        });
+}
+
 #[divan::bench]
 fn bench_unmarshal_large(bencher: divan::Bencher) {
     bencher