Add sandboxed-workspace module for isolated dev environments

Provides isolated development environments using either VMs (microvm.nix)
or containers (systemd-nspawn) with a unified configuration interface.

Features:
- Unified options with required type field ("vm" or "container")
- Shared base configuration for networking, SSH, users, packages
- Automatic SSH host key generation and persistence
- Shell aliases for workspace management (start/stop/status/ssh)
- Automatic /etc/hosts entries for workspace hostnames
- restartIfChanged support for both VMs and containers
- Passwordless doas in workspaces

Container backend:
- Uses hostBridge for proper bridge networking with /24 subnet
- systemd-networkd for IP configuration
- systemd-resolved for DNS

VM backend:
- TAP interface with deterministic MAC addresses
- virtiofs shares for workspace directories
- vsock CID generation

common/network/default.nix

@@ -12,6 +12,7 @@ in
     ./ping.nix
     ./tailscale.nix
     ./vpn.nix
+    ./sandbox.nix
   ];
 
   options.networking.ip_forward = mkEnableOption "Enable ip forwarding";

common/network/sandbox.nix

@@ -0,0 +1,115 @@
+{ config, lib, ... }:
+
+# Network configuration for sandboxed workspaces (VMs and containers)
+# Creates a bridge network with NAT for isolated environments
+
+with lib;
+
+let
+  cfg = config.networking.sandbox;
+in
+{
+  options.networking.sandbox = {
+    enable = mkEnableOption "sandboxed workspace network bridge";
+
+    bridgeName = mkOption {
+      type = types.str;
+      default = "sandbox-br";
+      description = "Name of the bridge interface for sandboxed workspaces";
+    };
+
+    subnet = mkOption {
+      type = types.str;
+      default = "192.168.83.0/24";
+      description = "Subnet for sandboxed workspace network";
+    };
+
+    hostAddress = mkOption {
+      type = types.str;
+      default = "192.168.83.1";
+      description = "Host address on the sandbox bridge";
+    };
+
+    upstreamInterface = mkOption {
+      type = types.str;
+      description = "Upstream network interface for NAT";
+    };
+  };
+
+  config = mkIf cfg.enable {
+    networking.ip_forward = true;
+
+    # Create the bridge interface
+    systemd.network.netdevs."10-${cfg.bridgeName}" = {
+      netdevConfig = {
+        Kind = "bridge";
+        Name = cfg.bridgeName;
+      };
+    };
+
+    systemd.network.networks."10-${cfg.bridgeName}" = {
+      matchConfig.Name = cfg.bridgeName;
+      networkConfig = {
+        Address = "${cfg.hostAddress}/24";
+        DHCPServer = false;
+        IPv4Forwarding = true;
+        IPv6Forwarding = false;
+        IPMasquerade = "ipv4";
+      };
+      linkConfig.RequiredForOnline = "no";
+    };
+
+    # Automatically attach VM tap interfaces to the bridge
+    systemd.network.networks."11-vm" = {
+      matchConfig.Name = "vm-*";
+      networkConfig.Bridge = cfg.bridgeName;
+      linkConfig.RequiredForOnline = "no";
+    };
+
+    # Automatically attach container veth interfaces to the bridge
+    systemd.network.networks."11-container" = {
+      matchConfig.Name = "ve-*";
+      networkConfig.Bridge = cfg.bridgeName;
+      linkConfig.RequiredForOnline = "no";
+    };
+
+    # NAT configuration for sandboxed workspaces
+    networking.nat = {
+      enable = true;
+      internalInterfaces = [ cfg.bridgeName ];
+      externalInterface = cfg.upstreamInterface;
+    };
+
+    # Enable systemd-networkd (required for bridge setup)
+    systemd.network.enable = true;
+
+    # When NetworkManager handles primary networking, disable systemd-networkd-wait-online.
+    # The bridge is the only interface managed by systemd-networkd and it never reaches
+    # "online" state without connected workspaces. NetworkManager-wait-online.service already
+    # gates network-online.target for the primary interface.
+    # On pure systemd-networkd systems (no NM), we just ignore the bridge.
+    systemd.network.wait-online.enable =
+      !config.networking.networkmanager.enable;
+    systemd.network.wait-online.ignoredInterfaces =
+      lib.mkIf (!config.networking.networkmanager.enable) [ cfg.bridgeName ];
+
+    # If NetworkManager is enabled, tell it to ignore sandbox interfaces
+    # This allows systemd-networkd and NetworkManager to coexist
+    networking.networkmanager.unmanaged = [
+      "interface-name:${cfg.bridgeName}"
+      "interface-name:vm-*"
+      "interface-name:ve-*"
+    ];
+
+    # Make systemd-resolved listen on the bridge for workspace DNS queries.
+    # By default resolved only listens on 127.0.0.53 (localhost).
+    # DNSStubListenerExtra adds the bridge address so workspaces can use the host as DNS.
+    services.resolved.settings.Resolve.DNSStubListenerExtra = cfg.hostAddress;
+
+    # Allow DNS traffic from workspaces to the host
+    networking.firewall.interfaces.${cfg.bridgeName} = {
+      allowedTCPPorts = [ 53 ];
+      allowedUDPPorts = [ 53 ];
+    };
+  };
+}