Chromecast discovery broken when using exit node in 1.98.x (regression from 1.96.3)

### What is the issue?

Description:
Chromecast devices are not discoverable when an exit node is active in Tailscale 1.98.x. This is a regression — the same setup works correctly on 1.96.3.

**Steps to reproduce:**
1. Install Tailscale 1.98.x on Windows
2. Enable an exit node (Raspberry Pi running Tailscale 1.96.4)
3. Enable "Allow local network access"
4. Attempt to discover Chromecast on the local network

**Expected behavior:**
Chromecast is discoverable on the local network while exit node is active, as it is on Tailscale 1.96.3.

**Actual behavior:**
Chromecast is not discoverable. Disabling the exit node immediately restores Chromecast discovery.

**Environment:**
- Client OS: Windows 11
- Tailscale client version: 1.98.x (broken), 1.96.3 (working)
- Exit node: Raspberry Pi, Tailscale 1.96.4, Linux 6.12.47+rpt-rpi-v8
- "Allow local network access": enabled

**Workaround:**
Downgrading to Tailscale 1.96.3 resolves the issue.

### Steps to reproduce

_No response_

### Are there any recent changes that introduced the issue?

_No response_

### OS

_No response_

### OS version

_No response_

### Tailscale version

_No response_

### Other software

_No response_

### Bug report

_No response_