perf(wire): pre-size effect batch encoding

hemx-core/benches/wire.rs

@@ -0,0 +1,51 @@
+//! Repeatable wire-encoding baseline: `cargo bench -p hemx-core --bench wire`.
+
+use hemx_core::{
+    BuildFingerprint, Effect, EffectBatch, Payload, ResourceId, ResourceKind, ResourceRef,
+    EFFECT_BATCH_ABI_VERSION,
+};
+use std::hint::black_box;
+use std::time::Instant;
+
+const ITERATIONS: u32 = 100_000;
+const ROUNDS: usize = 7;
+
+fn representative_batch() -> EffectBatch {
+    let target = ResourceRef::unscoped(ResourceId::new(ResourceKind::Slot, 42));
+    let ops = (0..50)
+        .map(|index| Effect::Put {
+            target: target.clone(),
+            payload: Payload::Text(format!("item-{index}-{}", "x".repeat(64))),
+        })
+        .collect();
+    EffectBatch {
+        abi_version: EFFECT_BATCH_ABI_VERSION,
+        fingerprint: BuildFingerprint(7),
+        ops,
+    }
+}
+
+fn main() {
+    let batch = representative_batch();
+    for _ in 0..10_000 {
+        black_box(batch.to_wire());
+    }
+
+    let mut nanos_per_batch = [0_u128; ROUNDS];
+    let mut checksum = 0_usize;
+    for round in &mut nanos_per_batch {
+        let start = Instant::now();
+        for _ in 0..ITERATIONS {
+            let bytes = black_box(&batch).to_wire();
+            checksum = checksum.wrapping_add(black_box(bytes.len()));
+        }
+        *round = start.elapsed().as_nanos() / u128::from(ITERATIONS);
+    }
+    nanos_per_batch.sort_unstable();
+
+    println!(
+        "median_ns_per_batch={} wire_len={} checksum={checksum}",
+        nanos_per_batch[ROUNDS / 2],
+        batch.encoded_len(),
+    );
+}