docs: Document the deployment

Add a top-level README mapping the boxes and a full docs/ tree: topic
pages (architecture, networking, deployment), per-site box pages for
colony and home with containers nested under their hosts, remote and
mobile boxes, the installer, and the home switch fabric reference
(folded in from home-switches.md, with AGENTS.md and code comments
retargeted to its new home). Box pages carry marked assignment tables
that CI regenerates from nixos.allAssignments.

AGENTS.md points at the new docs and keeps its terse agent version of
the mechanics, referring to the topic pages for depth.

docs/sites/colony/darts.md

@@ -0,0 +1,27 @@
+# darts
+
+An opaque third-party/customer VM. Declared in `colony`'s `my.vms.instances`
+but **not a NixOS system**: this repo knows nothing about what runs inside it
+and doesn't manage it.
+
+- **Source (host-side only):** the `darts` instance in
+  [`nixos/boxes/colony/vms/default.nix`](../../../nixos/boxes/colony/vms/default.nix)
+  and the `90-vm-darts` network in
+  [`nixos/boxes/colony/default.nix`](../../../nixos/boxes/colony/default.nix)
+- **Host:** VM on `colony`
+
+## Role
+
+- Customer/dedicated VM, left alone beyond hosting and connectivity.
+- 4 cores, 16 GiB RAM; a `root` LV plus `darts-media` and `darts-ext` LVs from
+  the `media`/`ext` volume groups.
+
+## Networking
+
+- Same customer-VM pattern as [`mail`](mail.md): dedicated unbridged TAP
+  (`vm-darts`, MAC `52:54:00:a8:29:cd`), point-to-point address
+  (`custRouting.darts-vm`) on the host side, link-routed public /32
+  `94.142.242.255`, and the IPv6 /64 `2a0e:97c0:4d2:2001::/64` with RAs.
+- DNS: `darts-cust.ams1.int.nul.ie`. Like the other customer prefixes, its
+  inbound traffic is accepted by `estuary` without per-port filtering and
+  forwarded on by `colony`.