Add lxc-tor-relay build type with nyx monitoring

New tor-relay build type (currently lxc-only) running a plain Tor
middle relay via modules/tor/enable-relay.nix, plus nyx for
interactive monitoring over the relay's control socket.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

README.md

@@ -10,11 +10,11 @@ pieces composed in `flake.nix`:
 
 - **Platforms** (what it runs on): `linode`, `proxmox`, `lxc`
 - **Build types** (what it's for): `minimal`, `nix-cache`, `server`, `docker`,
-  `gui`, `pxe-boot`, `tailscale-exit-node`
+  `gui`, `pxe-boot`, `tailscale-exit-node`, `tor-relay`
 
 Not every combination exists — `pxe-boot` has no `linode` variant, since
-PXE/DHCP/TFTP need LAN L2 adjacency that a Linode VPS doesn't have. The full
-list:
+PXE/DHCP/TFTP need LAN L2 adjacency that a Linode VPS doesn't have, and
+`tor-relay` currently only exists as `lxc-tor-relay`. The full list:
 
 | Target | Purpose |
 | --- | --- |
@@ -27,6 +27,7 @@ list:
 | `linode-gui` / `proxmox-gui` / `lxc-gui` | Cinnamon desktop workstation — previously the flat `nixos` target |
 | `proxmox-pxe-boot` / `lxc-pxe-boot` | HTTP/iPXE boot asset host — previously the flat `pxe-boot` target |
 | `linode-tailscale-exit-node` / `proxmox-tailscale-exit-node` / `lxc-tailscale-exit-node` | Tailscale exit node |
+| `lxc-tor-relay` | Tor middle relay |
 
 Which variant of a given buildtype is actually deployed isn't tracked
 anywhere in this repo — that's live infrastructure state, not something a