tailscale.com/wgengine: subscriber for `netmon.ChangeDelta` is slow

Found while reviewing [logs](http://logz0.corp.ts.net:9431/logs?start=2025-11-19T06%3A45%3A54Z&end=now&instances=610af4a80758fdce3d8b90ef60b5bda7247a6f1936d54e54a5aab87dff707e84&json=false&service=&match=is+slow) for reported panics on v1.90.6:
```
2025-11-19T04:38:35Z tailscale.com/wgengine userspace.go:542: subscriber for netmon.ChangeDelta is slow (31.9285352s elapsed)
2025-11-19T04:39:08Z tailscale.com/wgengine userspace.go:542: subscriber for netmon.ChangeDelta is slow (1m39.6118811s elapsed)
2025-11-19T04:39:34Z tailscale.com/wgengine userspace.go:542: subscriber for netmon.ChangeDelta is slow (25.2067014s elapsed)
2025-11-19T04:40:36Z tailscale.com/wgengine userspace.go:542: subscriber for netmon.ChangeDelta is slow (1m27.4083216s elapsed)
2025-11-19T04:42:34Z tailscale.com/wgengine userspace.go:542: subscriber for netmon.ChangeDelta is slow (1m57.6470489s elapsed)
2025-11-19T04:43:05Z tailscale.com/wgengine userspace.go:542: subscriber for netmon.ChangeDelta is slow (2m28.7588641s elapsed)
2025-11-19T04:43:45Z tailscale.com/wgengine userspace.go:542: subscriber for netmon.ChangeDelta is slow (39.9045116s elapsed)
```

11 minutes:
```
2025-11-19T05:48:39Z tailscale.com/wgengine userspace.go:542: subscriber for netmon.ChangeDelta is slow (11m17.4576416s elapsed)
```
etc.

cc @tailscale/client-core

Start in userspace.go around line 542, where the slow netmon.ChangeDelta subscriber warning is emitted. Review the subscriber path and the linked log examples to identify which operation blocks for tens of seconds or minutes. Done means the cause is addressed and the subscriber no longer produces these slow-handler warnings under the reported conditions.

Written by the indexing model from the issue text.