FR: Feature Request: Thin Edge Client for Embedded/IoT Devices (<256MB RAM)

### What are you trying to do?

Run Tailscale on embedded devices with <256MB RAM (routers, IoT gateways, industrial controllers, SBCs) where the current client's 72MB RSS footprint is too heavy.

**Current measurements (GL.iNet MT300N-V2, 128MB RAM, OpenWrt 24.10):**
```
VmRSS: 72,300 kB (56% of available RAM)
VmSize: 560,136 kB
Binary: ~25MB on disk (8MB compressed ipk)
```

On a 128MB device, Tailscale alone consumes **56% of RAM**, limiting what else can run. This is similar to #7272 where users hit OOM kills, but the root cause is baseline memory footprint.

**My use case:** Out-of-band management for remote lab infrastructure. The router provides rescue access to other devices when primary connectivity fails.

### How should we solve this?

Create a **"thin edge client"** variant optimized for resource-constrained gateways:

**Target:** <20MB RSS, <5MB binary

**Approach:**
- Leverage **kernel WireGuard** when available (vs userspace wireguard-go)
- Strip coordination-heavy features not needed for edge nodes
- Focus on: tunnel establishment, subnet routing, basic mesh participation
- Minimal dependencies

**Potential architecture:** Separate binary that handles data plane (tunneling) while delegating control plane to a more capable node or using simplified coordination protocol.

### What is the impact of not solving this?

**Blocked markets:**
- Millions of home routers (GL.iNet, Mikrotik, Ubiquiti EdgeRouter, TP-Link)
- IoT gateways with <256MB RAM
- Industrial edge controllers
- Legacy set-top boxes / Smart TVs
- Cost-sensitive deployments

**Current workarounds:**
1. Use Nebula instead (~5-10MB RSS) - loses Tailscale's ease-of-use
2. Use Headscale - same client footprint problem
3. Plain WireGuard - loses mesh coordination benefits

**Strategic impact:** These devices are ideal Tailscale nodes (always-on, gateway position, stable infrastructure) that drive network effects. Avery mentioned exploring machine-to-machine/IoT pricing in recent interviews - a thin client would enable this market.

### Anything else?

I've been a software engineer for 20+ years (founded an ISP, built traffic accounting systems, led teams at DAZN/Concur optimizing for resource-constrained platforms). Happy to contribute to this effort if it aligns with Tailscale's roadmap.

**References:**
- #7272 - OOM kills on 128MB OpenWRT devices (ongoing issue)
- [Recent Stratechery interview](https://stratechery.com/2025/an-interview-with-tailscale-co-founder-and-ceo-avery-pennarun/) where Avery discussed IoT/machine-to-machine market opportunity, which would benefit from smaller footprint edge clients