FN-6255: redirect tmpdir mkdtemp calls in tests

Keep test-created temp directories under the Fusion worker root.

- Redirect fs.mkdtemp and fs.promises.mkdtemp prefixes rooted at the OS temp dir into per-process worker sinks.
- Sweep stale redirect sinks and clean current-process sinks on exit.
- Add regression coverage for sync, async, realpath, nested, and Buffer prefix behavior.
- Remove the restored merger file-scope invariant test from quarantine.

Files changed:
 packages/core/src/__test-utils__/vitest-setup.ts   | 119 +++++++++++++++++++--
 .../__tests__/vitest-setup-tmp-redirect.test.ts    |  68 ++++++++++++
 scripts/lib/test-quarantine.json                   |   5 -
 3 files changed, 181 insertions(+), 11 deletions(-)

Fusion-Task-Id: FN-6255

Fusion-Task-Lineage: 4cc855c6-37dd-4dfb-a545-1fd1885779c4

packages/core/src/__test-utils__/vitest-setup.ts

@@ -15,7 +15,7 @@
 import { afterEach, expect } from "vitest";
 import { createRequire, syncBuiltinESMExports } from "node:module";
 import { tmpdir } from "node:os";
-import { dirname, join, resolve } from "node:path";
+import { basename, dirname, join, resolve } from "node:path";
 import { promisify } from "node:util";
 import { isMainThread } from "node:worker_threads";
 import { assertOutsideRealFusionPath } from "../test-safety.js";
@@ -40,7 +40,16 @@ const requireFromHere = createRequire(import.meta.url);
 const fs = requireFromHere("node:fs") as FsModule;
 const fsPromises = requireFromHere("node:fs/promises") as FsPromisesModule;
 const childProcess = requireFromHere("node:child_process") as ChildProcessModule;
-const { mkdtempSync, mkdirSync, rmSync, realpathSync, existsSync } = fs;
+const {
+  appendFileSync,
+  mkdtempSync,
+  mkdirSync,
+  readFileSync,
+  rmSync,
+  realpathSync,
+  existsSync,
+  writeFileSync,
+} = fs;
 
 type EmitWarningArgs = Parameters<typeof process.emitWarning>;
 type EmitWarningRestArgs = EmitWarningArgs extends [string | Error, ...infer Rest] ? Rest : never;
@@ -160,6 +169,102 @@ const WORKER_ROOT = join(tmpdir(), "fusion-test-workers");
 try { mkdirSync(WORKER_ROOT, { recursive: true }); } catch { /* ignore */ }
 process.env.FUSION_TEST_WORKER_ROOT = WORKER_ROOT;
 
+const REAL_TMPDIR = (() => {
+  try {
+    return realpathSync(tmpdir());
+  } catch {
+    return resolve(tmpdir());
+  }
+})();
+
+const TMPDIR_REDIRECT_REGISTRY = join(WORKER_ROOT, ".redir-pids");
+let tmpdirRedirectSink: string | null = null;
+let tmpdirRedirectExitCleanupInstalled = false;
+let tmpdirRedirectSweepComplete = false;
+
+function isProcessAlive(pid: number): boolean {
+  try {
+    process.kill(pid, 0);
+    return true;
+  } catch (error) {
+    const code = (error as NodeJS.ErrnoException).code;
+    return code === "EPERM";
+  }
+}
+
+function sweepDeadTmpdirRedirectSinks(): void {
+  if (tmpdirRedirectSweepComplete) return;
+  tmpdirRedirectSweepComplete = true;
+
+  let ownerPids: number[];
+  try {
+    ownerPids = Array.from(new Set(
+      readFileSync(TMPDIR_REDIRECT_REGISTRY, "utf8")
+        .split(/\r?\n/)
+        .map((line) => Number.parseInt(line, 10))
+        .filter((pid) => Number.isInteger(pid) && pid > 0),
+    ));
+  } catch {
+    return;
+  }
+
+  const liveOwnerPids: number[] = [];
+  for (const ownerPid of ownerPids) {
+    if (ownerPid === process.pid || isProcessAlive(ownerPid)) {
+      liveOwnerPids.push(ownerPid);
+      continue;
+    }
+
+    try {
+      rmSync(join(WORKER_ROOT, `redir-${ownerPid}`), { recursive: true, force: true });
+    } catch {
+      // Ignore stale-sink cleanup failures; global teardown still owns WORKER_ROOT.
+    }
+  }
+
+  try {
+    writeFileSync(TMPDIR_REDIRECT_REGISTRY, liveOwnerPids.length > 0 ? `${liveOwnerPids.join("\n")}\n` : "");
+  } catch {
+    // Best-effort only; stale entries are harmless and swept by future workers.
+  }
+}
+
+function ensureTmpdirRedirectSink(): string {
+  if (tmpdirRedirectSink) return tmpdirRedirectSink;
+
+  sweepDeadTmpdirRedirectSinks();
+  const sink = join(WORKER_ROOT, `redir-${process.pid}`);
+  mkdirSync(sink, { recursive: true });
+  try {
+    appendFileSync(TMPDIR_REDIRECT_REGISTRY, `${process.pid}\n`);
+  } catch {
+    // Best-effort only; the process exit hook and global teardown still clean up.
+  }
+  tmpdirRedirectSink = sink;
+
+  if (!tmpdirRedirectExitCleanupInstalled) {
+    tmpdirRedirectExitCleanupInstalled = true;
+    process.once("exit", () => {
+      try {
+        rmSync(sink, { recursive: true, force: true });
+      } catch {
+        // Best-effort only. vitest globalTeardown also sweeps WORKER_ROOT.
+      }
+    });
+  }
+
+  return sink;
+}
+
+function redirectTmpdirPrefix<T>(prefix: T): T {
+  if (typeof prefix !== "string") return prefix;
+
+  const parent = dirname(prefix);
+  if (parent !== tmpdir() && parent !== REAL_TMPDIR) return prefix;
+
+  return join(ensureTmpdirRedirectSink(), basename(prefix)) as T;
+}
+
 function ensureIsolatedHome(): void {
   const existingHome = process.env.HOME ?? process.env.USERPROFILE;
   if (existingHome && existingHome.includes(tmpdir()) && existingHome.includes(TEST_HOME_PREFIX)) {
@@ -290,8 +395,9 @@ function installFsGuards(): void {
     return originalFs.cpSync(src, dest, options as Parameters<typeof fs.cpSync>[2]);
   }) as typeof fs.cpSync;
   mutableFs.mkdtempSync = ((prefix, options) => {
-    guardOne(prefix, "fs.mkdtempSync");
-    return originalFs.mkdtempSync(prefix, options as Parameters<typeof fs.mkdtempSync>[1]);
+    const redirectedPrefix = redirectTmpdirPrefix(prefix);
+    guardOne(redirectedPrefix, "fs.mkdtempSync");
+    return originalFs.mkdtempSync(redirectedPrefix, options as Parameters<typeof fs.mkdtempSync>[1]);
   }) as typeof fs.mkdtempSync;
   mutableFs.openSync = ((path, flags, mode) => {
     guardOne(path, "fs.openSync");
@@ -408,8 +514,9 @@ function installFsGuards(): void {
     return originalFsPromises.open(...args);
   }) as typeof fsPromises.open;
   mutableFsPromises.mkdtemp = (async (...args: Parameters<typeof fsPromises.mkdtemp>) => {
-    guardOne(args[0], "fs.promises.mkdtemp");
-    return originalFsPromises.mkdtemp(...args);
+    const redirectedPrefix = redirectTmpdirPrefix(args[0]);
+    guardOne(redirectedPrefix, "fs.promises.mkdtemp");
+    return originalFsPromises.mkdtemp(redirectedPrefix, args[1]);
   }) as typeof fsPromises.mkdtemp;
   mutableFsPromises.truncate = (async (...args: Parameters<typeof fsPromises.truncate>) => {
     guardOne(args[0], "fs.promises.truncate");

packages/core/src/__tests__/vitest-setup-tmp-redirect.test.ts

@@ -0,0 +1,68 @@
+import { mkdtempSync, mkdirSync, rmSync, realpathSync } from "node:fs";
+import { mkdtemp } from "node:fs/promises";
+import { tmpdir } from "node:os";
+import { dirname, join, sep } from "node:path";
+import { afterEach, describe, expect, it } from "vitest";
+
+const createdPaths: string[] = [];
+
+function remember(path: string): string {
+  createdPaths.push(path);
+  return path;
+}
+
+function expectUnderWorkerRoot(path: string): void {
+  const workerRoot = process.env.FUSION_TEST_WORKER_ROOT;
+  expect(workerRoot).toBeTruthy();
+  expect(path.startsWith(`${workerRoot}${sep}`)).toBe(true);
+  expect(dirname(path)).toBe(join(workerRoot!, `redir-${process.pid}`));
+}
+
+afterEach(() => {
+  for (const path of createdPaths.splice(0).reverse()) {
+    rmSync(path, { recursive: true, force: true });
+  }
+});
+
+describe("vitest setup tmpdir mkdtemp redirect", () => {
+  it("redirects sync mkdtemp prefixes rooted directly at the OS temp dir", () => {
+    const path = remember(mkdtempSync(join(tmpdir(), "fn-redirect-sync-")));
+
+    expectUnderWorkerRoot(path);
+  });
+
+  it("redirects async mkdtemp prefixes rooted directly at the OS temp dir", async () => {
+    const path = remember(await mkdtemp(join(tmpdir(), "fn-redirect-async-")));
+
+    expectUnderWorkerRoot(path);
+  });
+
+  it("redirects the realpath spelling of the OS temp dir when it differs", () => {
+    const realTmpdir = realpathSync(tmpdir());
+    if (realTmpdir === tmpdir()) {
+      expect(realTmpdir).toBe(tmpdir());
+      return;
+    }
+
+    const path = remember(mkdtempSync(join(realTmpdir, "fn-redirect-realpath-")));
+
+    expectUnderWorkerRoot(path);
+  });
+
+  it("leaves nested temp-root prefixes unchanged", () => {
+    const parent = remember(join(tmpdir(), `fn-redirect-parent-${process.pid}-${Date.now()}`));
+    mkdirSync(parent, { recursive: true });
+
+    const path = remember(mkdtempSync(join(parent, "nested-")));
+
+    expect(path.startsWith(`${parent}${sep}`)).toBe(true);
+  });
+
+  it("leaves non-string prefixes untouched", () => {
+    const prefix = Buffer.from(join(tmpdir(), "fn-redirect-buffer-"));
+
+    const path = remember(mkdtempSync(prefix));
+
+    expect(path.startsWith(`${tmpdir()}${sep}`)).toBe(true);
+  });
+});

scripts/lib/test-quarantine.json

@@ -11,11 +11,6 @@
       "reason": "Flake: vi.mock('node:child_process') occasionally doesn't take under workspace-concurrent runs, letting real git binary leak and report staged files unrelated to test scope (trips FileScopeViolationError). Same logic covered by real-git fixture tests in reliability-interactions/workflow-and-file-scope. FN-6206.",
       "quarantinedAt": "2026-06-10"
     },
-    {
-      "file": "packages/engine/src/__tests__/merger-file-scope-invariant.test.ts",
-      "reason": "Flake: same mock-contention mode as the sibling changeset-file test above (vi.mock('node:child_process') not taking under concurrent load). FN-6206.",
-      "quarantinedAt": "2026-06-10"
-    },
     {
       "file": "packages/engine/src/__tests__/merger-ai-cleanup.test.ts",
       "reason": "Flake observed during FN-6206 verification: `pruneExistingAiMergeWorktrees skips active-session paths` failed in full `pnpm --filter @fusion/engine test` runs while the file passed standalone, indicating suite-order/concurrency sensitivity. Follow-up FN-6207.",