perf: avoid loading full node:stream in internal/fs/streams.ts

Replace require('node:stream') with direct submodule requires:
- require('internal/streams/readable')
- require('internal/streams/writable')
- require('internal/streams/end-of-stream')

This avoids eagerly loading unused submodules (Transform, Duplex,
PassThrough, pipeline, operators, etc.) when accessing process.stdin,
fs.createReadStream, or fs.createWriteStream.

Benchmark shows ~0.5ms improvement in process.stdin startup cost.

bench/snippets/process-stdin-startup.mjs

@@ -0,0 +1,49 @@
+// Measures the startup cost of accessing process.stdin
+// Each iteration spawns a fresh subprocess to avoid module cache effects.
+import { spawnSync } from "bun";
+
+const BUN = process.execPath;
+const N = 50;
+
+function measure(label, code) {
+  const times = [];
+  for (let i = 0; i < N; i++) {
+    const result = spawnSync({
+      cmd: [BUN, "-e", code],
+      stdout: "pipe",
+      stderr: "pipe",
+    });
+    const t = parseFloat(new TextDecoder().decode(result.stdout));
+    if (!isNaN(t)) times.push(t);
+  }
+  times.sort((a, b) => a - b);
+  const median = times[Math.floor(times.length / 2)];
+  const mean = times.reduce((a, b) => a + b, 0) / times.length;
+  const min = times[0];
+  const max = times[times.length - 1];
+  const p95 = times[Math.floor(times.length * 0.95)];
+  console.log(
+    `${label.padEnd(30)} median=${median.toFixed(3)}ms  mean=${mean.toFixed(3)}ms  min=${min.toFixed(3)}ms  max=${max.toFixed(3)}ms  p95=${p95.toFixed(3)}ms  (n=${times.length})`,
+  );
+}
+
+console.log(`Bun: ${BUN}`);
+console.log(`Iterations: ${N}\n`);
+
+// Baseline: empty script (process startup overhead only)
+measure("baseline (empty)", `process.stdout.write(String(0))`);
+
+// Access process.stdin which triggers internal/fs/streams + node:stream loading
+measure("process.stdin", `const t=performance.now();process.stdin;process.stdout.write(String(performance.now()-t))`);
+
+// Also measure require("node:stream") directly
+measure(
+  "require('node:stream')",
+  `const t=performance.now();require('node:stream');process.stdout.write(String(performance.now()-t))`,
+);
+
+// Measure just require("node:fs").createReadStream (also loads internal/fs/streams)
+measure(
+  "fs.createReadStream",
+  `const t=performance.now();require('node:fs').createReadStream;process.stdout.write(String(performance.now()-t))`,
+);

src/js/internal/fs/streams.ts

@@ -1,6 +1,8 @@
 // fs.ReadStream and fs.WriteStream are lazily loaded to avoid importing 'node:stream' until required
 import type { FileSink } from "bun";
-const { Readable, Writable, finished } = require("node:stream");
+const Readable = require("internal/streams/readable");
+const Writable = require("internal/streams/writable");
+const { finished } = require("internal/streams/end-of-stream");
 const fs: typeof import("node:fs") = require("node:fs");
 const { read, write, fsync, writev } = fs;
 const { FileHandle, kRef, kUnref, kFd } = (fs.promises as any).$data as {