Add read-update-write benchmarks

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

upb_test/hackers_bench.c

@@ -342,6 +342,34 @@ static void fn_count_all_crew(void *state) {
     upb_Arena_Free(arena);
 }
 
+static void fn_read_update_write(void *state) {
+    BenchData *d = state;
+    upb_Arena *arena = upb_Arena_New();
+    Campaign  *c     = Campaign_parse((const char *)d->data, d->len, arena);
+
+    upb_StringView name = { .data = "updated", .size = 7 };
+    Campaign_set_name(c, name);
+
+    size_t len;
+    char *out = Campaign_serialize(c, arena, &len);
+    g_sink = (uintptr_t)out;
+
+    upb_Arena_Free(arena);
+}
+
+static void bench_read_update_write(void) {
+    const char *sizes[] = {"tiny", "small", "medium", NULL};
+    printf("\n=== read_update_write ===\n");
+    for (int i = 0; sizes[i]; i++) {
+        BenchData d;
+        if (!load_bench_data(&d, sizes[i])) continue;
+        char label[80];
+        snprintf(label, sizeof(label), "Campaign_parse+set+serialize/%s  [%zu B]", sizes[i], d.len);
+        run_bench(fn_read_update_write, &d, d.len, label);
+        free_bench_data(&d);
+    }
+}
+
 static void bench_iterate(void) {
     const char *sizes[] = {"tiny", "small", "medium", NULL};
     printf("\n=== iterate ===\n");
@@ -374,6 +402,7 @@ int main(void) {
     bench_deep_parse();
     bench_field_access();
     bench_iterate();
+    bench_read_update_write();
 
     printf("\n");
     return 0;