Tailscale TCP direct connection packet loss high

### What is the issue?

I appear to be getting high packet loss on a direct TCP connection. This started somewhat recently, could something have broken in an update?

This is making using vscode remote ssh very painful because the connection will randomly hang for a second while typing in the terminal.

### Steps to reproduce

```
$ tailscale status
...
100.78.1.84 tagged-devices linux active; direct :41641
```

iperf3 directly to the other machine

```
$ iperf3 -c
Connecting to host , port 5201
[ 5] local 192.168.1.56 port 45606 connected to port 5201
[ ID] Interval Transfer Bitrate Retr Cwnd
[ 5] 0.00-1.00 sec 222 MBytes 1.86 Gbits/sec 0 3.90 MBytes
[ 5] 1.00-2.00 sec 244 MBytes 2.04 Gbits/sec 0 3.90 MBytes
[ 5] 2.00-3.00 sec 246 MBytes 2.07 Gbits/sec 0 3.90 MBytes
[ 5] 3.00-4.00 sec 248 MBytes 2.08 Gbits/sec 0 3.90 MBytes
[ 5] 4.00-5.00 sec 245 MBytes 2.06 Gbits/sec 0 3.90 MBytes
[ 5] 5.00-6.00 sec 249 MBytes 2.09 Gbits/sec 0 3.90 MBytes
[ 5] 6.00-7.00 sec 246 MBytes 2.06 Gbits/sec 0 3.90 MBytes
[ 5] 7.00-8.00 sec 249 MBytes 2.09 Gbits/sec 0 3.90 MBytes
[ 5] 8.00-9.00 sec 250 MBytes 2.10 Gbits/sec 0 3.90 MBytes
[ 5] 9.00-10.00 sec 250 MBytes 2.09 Gbits/sec 0 3.90 MBytes
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval Transfer Bitrate Retr
[ 5] 0.00-10.00 sec 2.39 GBytes 2.05 Gbits/sec 0 sender
[ 5] 0.00-10.04 sec 2.39 GBytes 2.05 Gbits/sec receiver

iperf Done.
```

iperf3 over tailscale

```
$ iperf3 -c 100.78.1.84
Connecting to host 100.78.1.84, port 5201
[ 5] local 100.100.153.126 port 43100 connected to 100.78.1.84 port 5201
[ ID] Interval Transfer Bitrate Retr Cwnd
[ 5] 0.00-1.00 sec 21.0 MBytes 176 Mbits/sec 17 9.59 KBytes
[ 5] 1.00-2.00 sec 17.9 MBytes 150 Mbits/sec 2866 1.20 KBytes
[ 5] 2.00-3.00 sec 0.00 Bytes 0.00 bits/sec 1021 614 KBytes
[ 5] 3.00-4.00 sec 17.8 MBytes 149 Mbits/sec 1796 1.20 KBytes
[ 5] 4.00-5.00 sec 17.8 MBytes 149 Mbits/sec 2622 2.92 MBytes
[ 5] 5.00-6.00 sec 0.00 Bytes 0.00 bits/sec 2 1.20 KBytes
[ 5] 6.00-7.00 sec 17.2 MBytes 145 Mbits/sec 2494 1.20 KBytes
[ 5] 7.00-8.00 sec 0.00 Bytes 0.00 bits/sec 1 1.20 KBytes
[ 5] 8.00-9.00 sec 17.1 MBytes 144 Mbits/sec 2408 1.20 KBytes
[ 5] 9.00-10.00 sec 0.00 Bytes 0.00 bits/sec 1 1.20 KBytes
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval Transfer Bitrate Retr
[ 5] 0.00-10.00 sec 109 MBytes 91.2 Mbits/sec 13228 sender
[ 5] 0.00-10.05 sec 106 MBytes 88.1 Mbits/sec receiver

iperf Done.
```

Client version

```
$ tailscale version
1.86.2
tailscale commit: c47caa10d6268583103fd4363792f577a584492c
long version: 1.86.2-tc47caa10d-gf5d087d04
other commit: f5d087d0447069b01b0b15ce03b54419a210d405
go version: go1.24.4
```

Server version

```
$ tailscale version
1.86.2
tailscale commit: c47caa10d6268583103fd4363792f577a584492c
long version: 1.86.2-tc47caa10d-gf5d087d04
other commit: f5d087d0447069b01b0b15ce03b54419a210d405
go version: go1.24.4
```

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

_No response_

### OS

Linux

### OS version

_No response_

### Tailscale version

1.86.2

### Other software

_No response_

### Bug report

BUG-4db74f1f750d8565a1ae905f81135a27f01bce053d5da64af35c5ee2af33e0e5-20250928222023Z-d567e7bda90efbd1