cassini: ship it

this fixes the firewall, but looks like the data format may have changed since upstream authored the project...

i think i can change:
```py
desc['Data']['Attributes']['MainboardID']
```
to
```
desc['Data']['MainboardID']
```

hosts/common/programs/assorted.nix

@@ -222,6 +222,7 @@ in
 
     pcTuiApps = declPackageSet [
       "aerc"  # email client
+      "cassini"  # Elegoo printer control. need here especially, for opening firewalls.
       # "msmtp"  # sendmail
       # "offlineimap"  # email mailbox sync
       # "sfeed"  # RSS fetcher

hosts/common/programs/cassini.nix

@@ -0,0 +1,28 @@
+{ config, lib, pkgs, ...}:
+let
+  cfg = config.sane.programs.cassini;
+in
+{
+  sane.programs.cassini = {
+    sandbox.method = null;  #< TODO: sandbox
+  };
+
+  # inspired by SSDP firewall code.
+  # Elegoo printers use their own SSDP-like discovery method, but on port 3000 instead of 1900 and 255.255.255.255 instead of 239.255.255.250:
+  # 1. i send a broadcast packet to 255.255.255.255 port 3000;
+  # 2. printers respond with a packet that originates from their port 3000, addressed to whichever port i sent from.
+  #
+  # TODO: can i generalize the SSDP rule from <hosts/common/net/upnp.nix> to be generic over port?
+  networking.firewall.extraCommands = with pkgs; lib.mkIf cfg.enabled ''
+    # originally for SSDP: <https://serverfault.com/a/911286>
+    # ipset -! means "don't fail if set already exists"
+    ${ipset}/bin/ipset create -! upnp hash:ip,port timeout 10
+    ${iptables}/bin/iptables -A OUTPUT -d 255.255.255.255/32 -p udp -m udp --dport 3000 -j SET --add-set upnp src,src --exist
+    ${iptables}/bin/iptables -A INPUT -p udp -m set --match-set upnp dst,dst -j ACCEPT
+
+    # IPv6 ruleset. ff02::/16 means *any* link-local multicast group (so this is probably more broad than it needs to be)
+    ${ipset}/bin/ipset create -! upnp6 hash:ip,port timeout 10 family inet6
+    ${iptables}/bin/ip6tables -A OUTPUT -d ff02::/16 -p udp -m udp --dport 3000 -j SET --add-set upnp6 src,src --exist
+    ${iptables}/bin/ip6tables -A INPUT -p udp -m set --match-set upnp6 dst,dst -j ACCEPT
+  '';
+}

hosts/common/programs/default.nix

@@ -28,6 +28,7 @@
     ./capsh.nix
     ./captree.nix
     ./cargo.nix
+    ./cassini.nix
     ./catt.nix
     ./celeste64.nix
     ./chatty.nix