fix(timers): add _idleStart property to Timeout object (#26021)

## Summary

- Add `_idleStart` property (getter/setter) to the Timeout object
returned by `setTimeout()` and `setInterval()`
- The property returns a monotonic timestamp (in milliseconds)
representing when the timer was created
- This mimics Node.js's behavior where `_idleStart` is the libuv
timestamp at timer creation time

## Test plan

- [x] Verified test fails with `USE_SYSTEM_BUN=1 bun test
test/regression/issue/25639.test.ts`
- [x] Verified test passes with `bun bd test
test/regression/issue/25639.test.ts`
- [x] Manual verification:
  ```bash
  # Bun with fix - _idleStart exists
./build/debug/bun-debug -e "const t = setTimeout(() => {}, 0);
console.log('_idleStart' in t, typeof t._idleStart); clearTimeout(t)"
  # Output: true number
  
  # Node.js reference - same behavior
node -e "const t = setTimeout(() => {}, 0); console.log('_idleStart' in
t, typeof t._idleStart); clearTimeout(t)"
  # Output: true number
  ```

Closes #25639

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Bot <claude-bot@bun.sh>
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

test/regression/issue/25639.test.ts

@@ -0,0 +1,64 @@
+import { expect, test } from "bun:test";
+
+// GitHub Issue #25639: setTimeout Timeout object missing _idleStart property
+// Next.js 16 uses _idleStart to coordinate timers for Cache Components
+
+test("setTimeout returns Timeout object with _idleStart property", () => {
+  const timer = setTimeout(() => {}, 100);
+
+  try {
+    // Verify _idleStart exists and is a number
+    expect("_idleStart" in timer).toBe(true);
+    expect(typeof timer._idleStart).toBe("number");
+
+    // _idleStart should be a positive timestamp
+    expect(timer._idleStart).toBeGreaterThan(0);
+  } finally {
+    clearTimeout(timer);
+  }
+});
+
+test("setInterval returns Timeout object with _idleStart property", () => {
+  const timer = setInterval(() => {}, 100);
+
+  try {
+    // Verify _idleStart exists and is a number
+    expect("_idleStart" in timer).toBe(true);
+    expect(typeof timer._idleStart).toBe("number");
+
+    // _idleStart should be a positive timestamp
+    expect(timer._idleStart).toBeGreaterThan(0);
+  } finally {
+    clearInterval(timer);
+  }
+});
+
+test("_idleStart is writable (Next.js modifies it to coordinate timers)", () => {
+  const timer = setTimeout(() => {}, 100);
+
+  try {
+    const originalIdleStart = timer._idleStart;
+    expect(typeof originalIdleStart).toBe("number");
+
+    // Next.js sets _idleStart to coordinate timers
+    const newIdleStart = originalIdleStart - 100;
+    timer._idleStart = newIdleStart;
+    expect(timer._idleStart).toBe(newIdleStart);
+  } finally {
+    clearTimeout(timer);
+  }
+});
+
+test("timers created at different times have different _idleStart values", async () => {
+  const timer1 = setTimeout(() => {}, 100);
+  // Wait a bit to ensure different timestamp
+  await Bun.sleep(10);
+  const timer2 = setTimeout(() => {}, 100);
+
+  try {
+    expect(timer2._idleStart).toBeGreaterThanOrEqual(timer1._idleStart);
+  } finally {
+    clearTimeout(timer1);
+    clearTimeout(timer2);
+  }
+});