test: VM smoke and Lightning regtest suites

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/regtest-node.nix

@@ -0,0 +1,59 @@
+# regtest-node — shared bitcoind(regtest) + Core Lightning + LND config.
+#
+# The single source of truth for the regtest stack, imported by BOTH:
+#   - the CI test       → tests/regtest-core.nix
+#   - the interactive VM → tests/regtest-interactive.nix (via flake.nix)
+# so the two can never drift.
+#
+# Consumers must ALSO import `nix-bitcoin.nixosModules.default` — it needs
+# the flake input, which a plain module can't reach on its own.
+#
+# Both LN nodes are co-located and share the local bitcoind; lnd's p2p
+# port is moved to 9736 so it doesn't clash with clightning's 9735. CLIs
+# run as the nix-bitcoin `operator` user.
+{
+  config,
+  pkgs,
+  lib,
+  ...
+}:
+{
+  nix-bitcoin.generateSecrets = true;
+  nix-bitcoin.operator.enable = true;
+
+  services.bitcoind = {
+    enable = true;
+    regtest = true;
+    # regtest has no fee estimation, so sendtoaddress/funding need a
+    # fallback fee. nix-bitcoin appends extraConfig inside the [regtest]
+    # section, so this applies to the regtest network.
+    extraConfig = "fallbackfee=0.0002";
+  };
+
+  # Pre-mine a funded "test" wallet on bitcoind startup. 110 > 100 so the
+  # coinbase outputs are mature/spendable.
+  # NOTE: keep this height in sync with the assertions in
+  # tests/regtest-core.nix (it checks the nodes reach block 110).
+  systemd.services.bitcoind.postStart = lib.mkAfter ''
+    cli=${config.services.bitcoind.cli}/bin/bitcoin-cli
+    if ! $cli listwallets | ${pkgs.jq}/bin/jq -e 'index("test")' >/dev/null; then
+      "$cli" -named createwallet wallet_name=test load_on_startup=true
+      addr=$("$cli" -rpcwallet=test getnewaddress)
+      "$cli" generatetoaddress 110 "$addr"
+    fi
+  '';
+
+  services.clightning.enable = true; # p2p :9735
+  services.lnd = {
+    enable = true;
+    port = 9736; # avoid clashing with clightning's 9735
+  };
+
+  # jq is used by the test script and the interactive helpers.
+  environment.systemPackages = [ pkgs.jq ];
+
+  # Headroom for bitcoind + two lightning daemons in one VM.
+  virtualisation.cores = 2;
+  virtualisation.memorySize = 3072;
+  virtualisation.diskSize = 8192;
+}