Add device architecture documentation and D2 diagram

- Add comprehensive DEVICE_ARCHITECTURE.md with detailed device specifications
- Create D2 diagram showing device relationships and network topology
- Organize assets in dedicated assets/ folder
- Document all devices: servers (mirai, deoxys, tsuba), workstations (ryu),
  and portable devices (kuro, shiro, SteamDeck)
- Include deployment flows, network architecture, and build infrastructure

assets/devices-diagram.d2

@@ -0,0 +1,235 @@
+title: Device Architecture Overview {
+  near: top-center
+  shape: text
+  style: {
+    font-size: 24
+    bold: true
+  }
+}
+
+# Device Groups
+servers: Server Infrastructure {
+  style.fill: "#e8f4fd"
+  style.stroke: "#1e3a8a"
+  style.stroke-width: 2
+
+  mirai: mirai {
+    shape: rectangle
+    style.fill: "#fbbf24"
+
+    label: "mirai\nMain Server\nx86_64-linux\nUser: fs0c131y\nHeadless"
+
+    services: Services {
+      shape: cylinder
+      style.fill: "#34d399"
+      label: "Services\n• Nextcloud\n• Gitea\n• Grafana\n• Minecraft\n• Immich\n• Paperless\n• Tailscale\n• And more..."
+    }
+  }
+
+  deoxys: deoxys {
+    shape: rectangle
+    style.fill: "#a78bfa"
+    label: "deoxys\nVM Server\nx86_64-linux\nUser: servius\nHeadless"
+  }
+
+  tsuba: tsuba {
+    shape: rectangle
+    style.fill: "#fb7185"
+    label: "tsuba\nRaspberry Pi\naarch64-linux\nUser: servius\nHeadless"
+  }
+}
+
+workstations: Development Workstations {
+  style.fill: "#f0fdf4"
+  style.stroke: "#15803d"
+  style.stroke-width: 2
+
+  ryu: ryu {
+    shape: rectangle
+    style.fill: "#3b82f6"
+    label: "ryu\nMain Desktop\nx86_64-linux\nUser: servius\nHyprland + GNOME"
+
+    monitors: Multi-Monitor Setup {
+      style.fill: "#ddd6fe"
+
+      primary: "HDMI-A-1\n(Gigabyte FO27Q3)" {
+        shape: rectangle
+        style.fill: "#c4b5fd"
+      }
+      secondary: "DP-3\n(Acer XV272U)" {
+        shape: rectangle
+        style.fill: "#c4b5fd"
+      }
+      tertiary: "DP-1\n(Gigabyte M27Q)" {
+        shape: rectangle
+        style.fill: "#c4b5fd"
+      }
+    }
+  }
+}
+
+mobile: Portable Devices {
+  style.fill: "#fef3c7"
+  style.stroke: "#d97706"
+  style.stroke-width: 2
+
+  kuro: kuro {
+    shape: rectangle
+    style.fill: "#6b7280"
+    label: "kuro\nMacBook\naarch64-darwin\nUser: fs0c131y"
+  }
+
+  shiro: shiro {
+    shape: rectangle
+    style.fill: "#9ca3af"
+    label: "shiro\nMacBook\naarch64-darwin\nUser: servius"
+  }
+
+  deck: SteamDeck {
+    shape: rectangle
+    style.fill: "#ef4444"
+    label: "SteamDeck\nGaming Handheld\nx86_64-linux\nUser: deck\nHome Manager Only"
+  }
+}
+
+# Network Infrastructure
+network: Network Infrastructure {
+  style.fill: "#fdf2f8"
+  style.stroke: "#db2777"
+  style.stroke-width: 2
+
+  tailscale: Tailscale VPN {
+    shape: cloud
+    style.fill: "#ec4899"
+  }
+
+  zerotier: ZeroTier Network {
+    shape: cloud
+    style.fill: "#f97316"
+  }
+
+  local_network: Local Network {
+    shape: cloud
+    style.fill: "#06b6d4"
+  }
+}
+
+# Build Infrastructure
+builders: Build Machines {
+  style.fill: "#f5f5f4"
+  style.stroke: "#525252"
+  style.stroke-width: 2
+
+  build_info: "Distributed Builds\nmirai, shiro (+ tsuba)" {
+    shape: text
+    style.font-size: 14
+    style.bold: true
+  }
+}
+
+# Configuration Management
+config_mgmt: Configuration Management {
+  style.fill: "#fefce8"
+  style.stroke: "#ca8a04"
+  style.stroke-width: 2
+
+  nixos_flake: NixOS Flake {
+    shape: hexagon
+    style.fill: "#facc15"
+  }
+
+  home_manager: Home Manager {
+    shape: hexagon
+    style.fill: "#eab308"
+  }
+
+  darwin: nix-darwin {
+    shape: hexagon
+    style.fill: "#f59e0b"
+  }
+}
+
+# Deployment Connections
+workstations.ryu -> servers.mirai: SSH Deploy {
+  style.stroke: "#059669"
+  style.stroke-width: 2
+}
+
+workstations.ryu -> servers.deoxys: SSH Deploy {
+  style.stroke: "#059669"
+  style.stroke-width: 2
+}
+
+workstations.ryu -> servers.tsuba: "SSH Deploy\n(tsuba.darksailor.dev)" {
+  style.stroke: "#059669"
+  style.stroke-width: 2
+}
+
+mobile.kuro -> servers.mirai: SSH Deploy {
+  style.stroke: "#7c3aed"
+  style.stroke-width: 2
+}
+
+mobile.kuro -> mobile.shiro: SSH Deploy {
+  style.stroke: "#7c3aed"
+  style.stroke-width: 2
+}
+
+workstations.ryu -> mobile.deck: "SSH Deploy\n(steamdeck)" {
+  style.stroke: "#dc2626"
+  style.stroke-width: 2
+}
+
+# Network connections
+servers.mirai -> network.tailscale
+servers.deoxys -> network.tailscale
+servers.tsuba -> network.tailscale
+workstations.ryu -> network.local_network
+mobile.deck -> network.tailscale
+
+servers.mirai -> network.zerotier
+workstations.ryu -> network.zerotier
+
+# Build relationships
+workstations.ryu -> builders: Uses distributed builds
+mobile.kuro -> builders: Uses distributed builds
+servers.mirai -> builders: Build server
+mobile.shiro -> builders: Build server
+
+# Configuration connections
+config_mgmt.nixos_flake -> servers.mirai
+config_mgmt.nixos_flake -> servers.deoxys
+config_mgmt.nixos_flake -> servers.tsuba
+config_mgmt.nixos_flake -> workstations.ryu
+
+config_mgmt.darwin -> mobile.kuro
+config_mgmt.darwin -> mobile.shiro
+
+config_mgmt.home_manager -> mobile.deck
+
+# Legend
+legend: Legend {
+  near: bottom-right
+  style.fill: "#f9fafb"
+  style.stroke: "#6b7280"
+
+  servers_legend: "🖥️ Servers (headless)" {
+    shape: text
+    style.font-size: 12
+  }
+
+  workstations_legend: "💻 Workstations (GUI)" {
+    shape: text
+    style.font-size: 12
+  }
+
+  mobile_legend: "📱 Portable/Mobile" {
+    shape: text
+    style.font-size: 12
+  }
+
+  deploy_legend: "→ SSH Deploy" {
+    shape: text
+    style.font-size: 12
+  }
+}