perf(scripts): emit a server module beside the runtime bundle entry (#16765)

The runtime bundle now also has a server entry point, dist-runtime/server.mjs: the bundled dist/index.js without the argv shim. index.mjs loads it, so importing server.mjs loads the server code without starting it, and a compile cache warmed from it covers exactly the files a boot loads. bundleRuntime reports it as serverModule.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

server/scripts/bundle-runtime.test.mjs

@@ -2,6 +2,7 @@ import { execFile } from "node:child_process";
 import { mkdirSync, mkdtempSync, realpathSync, rmSync, symlinkSync, writeFileSync } from "node:fs";
 import { tmpdir } from "node:os";
 import path from "node:path";
+import { pathToFileURL } from "node:url";
 import { promisify } from "node:util";
 
 import { afterEach, beforeEach, describe, expect, it } from "vitest";
@@ -84,6 +85,18 @@ describe("bundleRuntime", () => {
     expect(output.entryDir).toBe(path.join(root, "server", "dist"));
   });
 
+  it("emits a server module that loads without starting the server", async () => {
+    const outdir = path.join(root, "server", "dist-runtime");
+    const summary = await bundleRuntime({ entry: path.join(root, "server", "dist", "index.js"), outdir });
+    expect(summary.serverModule).toBe(path.join(outdir, "server.mjs"));
+    const probe = path.join(root, "server", "probe.mjs");
+    write(probe, `await import(${JSON.stringify(pathToFileURL(summary.serverModule).href)});\nconsole.log("loaded");\n`);
+    const loaded = await execFileAsync(process.execPath, [probe], { cwd: path.join(root, "server"), timeout: 30_000 });
+    expect(loaded.stdout.trim()).toBe("loaded");
+    const started = await execFileAsync(process.execPath, [summary.outfile], { cwd: path.join(root, "server"), timeout: 30_000 });
+    expect(JSON.parse(started.stdout).late).toBe(43);
+  });
+
   it("leaves the server's own dependencies to runtime resolution", async () => {
     const outdir = path.join(root, "server", "dist-runtime");
     const summary = await bundleRuntime({ entry: path.join(root, "server", "dist", "index.js"), outdir });