[Windows] Extremely low upload throughput via DERP compared to WSL on same host (10Mbps vs 40Mbps)

### What is the issue?

I am experiencing a significant performance disparity between the native Windows Tailscale client and the Tailscale client running inside WSL (Windows Subsystem for Linux) on the **same physical machine**.

**Setup:**
* **Topology:** Self-hosted DERP server used for relaying traffic. ACL configured to enforce DERP connections.
* **Destination:** A Debian MiniPC running `iperf3 -s`.
* **Source:** A Windows PC (acting as the sender).

**The Problem:**
When sending data from the **Windows native client** (via `iperf3 -c` or `python http.server` download), the throughput is capped at approximately **10 Mbps (700KB/s)**.

**The Control Group (WSL):**
When running Tailscale independently inside **WSL (Debian)** on the **same Windows host** (using the same physical network interface), the throughput stabilizes at **40 Mbps (~5.3 MB/s)**.

Since WSL shares the same physical link and DERP server, this indicates the issue lies specifically within the Windows client implementation or its interaction with the Windows network stack, rather than the network bandwidth or DERP node capacity.

### Steps to reproduce

1. Set up a self-hosted DERP server and ensure clients connect via this relay.
2. **Scenario A (Windows Native):**
* Run `iperf3 -c ` from PowerShell.
* **Result:** Throughput struggles to exceed 10 Mbps.
* (Verification): Host a file via `python -m http.server` on Windows. Download it from the remote Linux node via `wget`. Speed is ~700KB/s.
3. **Scenario B (WSL on same Host):**
* Install Tailscale inside WSL2 (Ubuntu-24.04).
* Run `iperf3 -c ` from WSL terminal.
* **Result:** Throughput reaches 40-60 Mbps.
* (Verification): Host a file via `python -m http.server` inside WSL. Download it from the remote Linux node. Speed is ~5.3MB/s.

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

No specific changes. This is a new setup testing DERP performance.

### OS

Windows

### OS version

Windows 11 24H2

### Tailscale version

1.90.8

### Other software

**Troubleshooting attempts taken (All failed to improve Windows performance):**
1. **RSC:** Disabled Receive Segment Coalescing via `netsh` and PowerShell.
2. **MTU:** Manually adjusted Tailscale interface MTU to 1200.
3. **TCP Auto-Tuning:** Set to `experimental` and `normal`.
4. **Congestion Control:** Tried changing to `ctcp` and `cubic`.
5. **Offloading:** Disabled UDP Checksum Offload via PowerShell.
6. **Registry:** Adjusted `NetworkThrottlingIndex` to `ffffffff`.
7. **Nagle's Algorithm:** Added `TcpAckFrequency=1` and `TCPNoDelay=1` to the Tailscale interface GUID in registry.
8. **QoS:** Disabled QoS Packet Scheduler on the virtual adapter.

Verified no "Game Booster" or "Network Manager" software (like cFosSpeed) is installed.

### Bug report

BUG-d43a2be8563ea8abe0ca2df1da83d56e14d7d4ed25a7df72e40819d6861ede9b-20251121064759Z-3663a48d2e863eba