Add tailscale-exit-node build type across all three platforms

New build type dedicated to Tailscale exit-node capability, wired up for
linode/proxmox/lxc like every other build type (the lxc variant is the one
actually intended for deployment). Kept separate from the "server" host
rather than bundling exit-node capability onto it.

Trimmed modules/tailscale/exit-node.nix down to pure exit-node behavior:
dropped the old --advertise-routes=${vars.lanCidr} bundling (meaningless
for a Linode-hosted VPS with no path to the LAN), and switched
extraUpFlags -> extraSetFlags. Confirmed against nixpkgs' tailscale.nix
that extraUpFlags is only applied by tailscaled-autoconnect, which itself
only runs when services.tailscale.authKeyFile is set -- nothing in this
repo sets one, so the old flags would never have actually been applied.
extraSetFlags runs unconditionally via tailscaled-set on every boot, so
--advertise-exit-node self-reapplies once the operator has done the
one-time manual `tailscale up` auth.

Verified: all three new targets eval cleanly, nixpkgs-fmt/statix clean,
and a dry-run build of lxc-tailscale-exit-node's tarball resolves its full
closure including tailscaled-set.service.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01La55Nsss8jZ7ZuzUV9mfot

README.md

@@ -10,7 +10,7 @@ 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`
+  `gui`, `pxe-boot`, `tailscale-exit-node`
 
 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
@@ -26,6 +26,7 @@ list:
 | `linode-docker` / `proxmox-docker` / `lxc-docker` | Docker host for the main container stack (`proxmox-docker` is the real, deployed one — previously the flat `docker` target) |
 | `linode-gui` / `proxmox-gui` / `lxc-gui` | Cinnamon desktop workstation (`proxmox-gui` is the real, deployed one — previously the flat `nixos` target) |
 | `proxmox-pxe-boot` / `lxc-pxe-boot` | HTTP/iPXE boot asset host (`proxmox-pxe-boot` is the real, deployed one — previously the flat `pxe-boot` target) |
+| `linode-tailscale-exit-node` / `proxmox-tailscale-exit-node` / `lxc-tailscale-exit-node` | Tailscale exit node (no deployed target yet; `lxc-tailscale-exit-node` is the one planned for actual use) |
 
 The "(real, deployed)" targets above are also tracked machine-readably in
 `variables.nix`'s `deployedTargets` — keep both in sync when a deployment