fix: harden preprocessing state and child capabilities

backend/src/workspaces/workspace-fs-at.ts

@@ -1,4 +1,6 @@
 import { createRequire } from "node:module";
+import { closeSync, openSync } from "node:fs";
+import { spawn } from "node:child_process";
 import type { WorkspaceFsAtBindingV1, NativeWorkspaceFsAtHandleV1, NativeWorkspaceFsAtStatV1, NativeWorkspaceFsAtComponentV1 } from "../native/workspace-fs-at-binding.js";
 const require = createRequire(import.meta.url);
 const binding = require("../../native/workspace-fs-at/build/Release/workspace_fs_at.node") as WorkspaceFsAtBindingV1 & {
@@ -70,3 +72,28 @@ export class WorkspaceFsAtV1 {
     withLockFd(owned, fd => fsExt.flockSync(fd, operation));
   }
 }
+
+
+/** Internal child boundary. It deliberately returns a ChildProcess result, never an FD. */
+export function spawnChildWithWorkspaceCapabilities(
+  writer: OwnedWorkspaceFsAtRegularFile,
+  root: OwnedWorkspaceFsAtDirectory,
+  executable: string,
+  args: readonly string[],
+  environment: NodeJS.ProcessEnv = process.env,
+): Promise<{ exitCode: number; stdout: Uint8Array; stderr: Uint8Array }> {
+  if (!rawHandles.has(writer) || !rawHandles.has(root)) throw new Error("preprocessing_conflict");
+  const writerRaw = rawHandles.get(writer)!; const rootRaw = rawHandles.get(root)!;
+  const opened: number[] = [];
+  try {
+    const writerFd = openSync("/dev/null", "r"); opened.push(writerFd);
+    const rootFd = openSync("/dev/null", "r"); opened.push(rootFd);
+    binding.duplicateForChildStdio(writerRaw, rootRaw, writerFd, rootFd);
+    const child = spawn(executable, [...args], { stdio: ["ignore", "pipe", "pipe", writerFd, rootFd], env: { ...environment, THOTH_WORKSPACE_CAPABILITY_REQUIRED: "1" } });
+    const out: Buffer[] = []; const err: Buffer[] = [];
+    child.stdout?.on("data", (chunk: Buffer) => out.push(chunk)); child.stderr?.on("data", (chunk: Buffer) => err.push(chunk));
+    return new Promise((resolve, reject) => {
+      child.once("error", reject); child.once("close", code => resolve({ exitCode: code ?? 1, stdout: Buffer.concat(out), stderr: Buffer.concat(err) }));
+    });
+  } finally { for (const fd of opened) { try { closeSync(fd); } catch {} } }
+}