Add read-update-write benchmarks

Add benchmarks for parsing, updating, and serializing in Rust and C.
Update the fork instructions in AGENTS.md.

benches/benches/hackers_bench.rs

@@ -205,11 +205,39 @@ fn bench_iterate(c: &mut Criterion) {
     group.finish();
 }
 
+/// Parse, update a field, and serialize back to a buffer.
+fn bench_read_update_write(c: &mut Criterion) {
+    let cases = [
+        ("tiny", load("tiny")),
+        ("small", load("small")),
+        ("medium", load("medium")),
+    ];
+
+    let mut group = c.benchmark_group("read_update_write");
+    for (label, maybe_data) in &cases {
+        let Some(data) = maybe_data else { continue };
+        group.throughput(Throughput::Bytes(data.len() as u64));
+        group.bench_with_input(BenchmarkId::new("Campaign", label), data, |b, data| {
+            b.iter(|| {
+                let campaign = Campaign::new(data).unwrap();
+                let mut buf = vec![0u8; data.len() * 2];
+                let res = CampaignBuilder::builder(&mut buf)
+                    .name("updated")
+                    .with(&campaign)
+                    .finish();
+                black_box(res)
+            })
+        });
+    }
+    group.finish();
+}
+
 criterion_group!(
     benches,
     bench_shallow_parse,
     bench_deep_parse,
     bench_field_access,
-    bench_iterate
+    bench_iterate,
+    bench_read_update_write
 );
 criterion_main!(benches);