feat(hosts): add the Raspberry Pi Zero 2 W Psion sidecar

A headless aarch64 companion for a Psion 5MX: PPP over RS232 with NAT out to
wifi and a telnet login, plus a cleartext POP3/SMTP proxy for the Psion's mail
client.

- hosts/PiZero2W/: host config, serial-ppp.nix, email-proxy.nix, an SD-image
  variant, and a hardware-configuration.nix placeholder.
- Host table entry on nixos-hardware's raspberry-pi-3 profile; the Zero 2 W is
  the Pi 3's BCM2837 SoC. nixpkgs' linuxPackages_rpi02w is deprecated and warns
  that the linux-rpi series is being removed in favour of nixos-hardware.
- The host owns its firmware partition (hardware.raspberry-pi.firmware), which
  is what puts the disable-bt and uart0/ctsrts overlays in config.txt so
  /dev/ttyAMA0 is the RS232 header rather than Bluetooth. uboot.enable keeps the
  U-Boot -> extlinux boot path the rewritten config.txt would otherwise lose.
- packages.aarch64-linux.zero2w-sd-image: the host's own configuration as an
  installable card. The board has no Ethernet and no free serial port, so a
  generic image would leave no way in.
- The mail proxy comes from the legacy-email-proxy flake, which provides the
  package and the NixOS module; nothing about it is vendored here.
- docs/hosts/pizero2w.md, plus README host table and shared-layer notes.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

flake.lock

@@ -185,6 +185,26 @@
         "type": "github"
       }
     },
+    "legacy-email-proxy": {
+      "inputs": {
+        "nixpkgs": [
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1787315211,
+        "narHash": "sha256-FuZ9nXMRtnMPO/wbjYkpsKn6K/FFc64P5XDmCyfyxGs=",
+        "ref": "refs/heads/main",
+        "rev": "f1e1373fd350fd77f1848eddfa67ed9e00724c25",
+        "revCount": 13,
+        "type": "git",
+        "url": "https://code.emmathe.dev/lyrathorpe/legacy-email-proxy"
+      },
+      "original": {
+        "type": "git",
+        "url": "https://code.emmathe.dev/lyrathorpe/legacy-email-proxy"
+      }
+    },
     "nix-darwin": {
       "inputs": {
         "nixpkgs": [
@@ -368,6 +388,7 @@
         "git-hooks": "git-hooks",
         "home-manager": "home-manager",
         "kube-tmux": "kube-tmux",
+        "legacy-email-proxy": "legacy-email-proxy",
         "nix-darwin": "nix-darwin",
         "nix-homebrew": "nix-homebrew",
         "nix-index-database": "nix-index-database",