improve: reduce whole-file write receipt latency (#148132)

* improve: reduce write receipt latency by reusing bounded diffs

* chore: expose write diff benchmark as a package script

scripts/bench-write-diff.ts

@@ -0,0 +1,66 @@
+// Run on baseline and candidate with: node --import ./scripts/tsx.mjs scripts/bench-write-diff.ts
+// Warm-cache write-tool timings, not model latency or fsync durability. Setup/assertions excluded.
+import assert from "node:assert/strict";
+import { mkdtemp, readFile, rm, writeFile } from "node:fs/promises";
+import os from "node:os";
+import path from "node:path";
+import { performance } from "node:perf_hooks";
+import { createWriteTool } from "../src/agents/sessions/tools/write.js";
+
+const samples = 40;
+const warmups = 8;
+const directory = await mkdtemp(path.join(os.tmpdir(), "openclaw-write-bench-"));
+const tool = createWriteTool(directory);
+const results = [];
+try {
+  for (const size of [1024, 65536, 1048576, 4194304]) {
+    const prefix = "target=0\n";
+    const line = "stable synthetic benchmark content 0123456789 abcdefghijklmnopqrstuvwxyz\n";
+    const before =
+      prefix + line.repeat(Math.ceil(size / line.length)).slice(0, size - prefix.length);
+    for (const scenario of ["small-change", "full-rewrite", "noop"] as const) {
+      const after =
+        scenario === "full-rewrite"
+          ? before.replaceAll("stable", "CHANGED").replace("target=0", "target=9")
+          : scenario === "small-change"
+            ? before.replace("target=0", "target=1")
+            : before;
+      const file = path.join(directory, "fixture.txt");
+      const timings: number[] = [];
+      for (let iteration = -warmups; iteration < samples; iteration++) {
+        await writeFile(file, before);
+        const start = performance.now();
+        await tool.execute("bench", { path: "fixture.txt", content: after });
+        const elapsed = performance.now() - start;
+        assert.equal(await readFile(file, "utf8"), after);
+        if (iteration >= 0) {
+          timings.push(elapsed);
+        }
+      }
+      const sorted = timings.toSorted((a, b) => a - b);
+      results.push({
+        size,
+        scenario,
+        medianMs: sorted[Math.floor(samples / 2)],
+        p95Ms: sorted[Math.ceil(samples * 0.95) - 1],
+        samplesMs: timings,
+      });
+    }
+  }
+  console.log(
+    JSON.stringify(
+      {
+        node: process.version,
+        cpu: os.cpus()[0]?.model,
+        samples,
+        warmups,
+        maxRssKiB: process.resourceUsage().maxRSS,
+        results,
+      },
+      null,
+      2,
+    ),
+  );
+} finally {
+  await rm(directory, { recursive: true, force: true });
+}