refactor: move vendor/nix-bitcoin to modules/bitcoin, remove overlays

- Move modules/vendor/nix-bitcoin/* -> modules/bitcoin/* (clean, Sovran-owned)
  * modules/bitcoin/default.nix imports the 6 tailored services
  * modules/bitcoin/common.nix bundles secrets/onion/lib
  * modules/bitcoin/stubs.nix kept minimal (no clightning)
  * packages/lndinit/default.nix replaces pkgs/sovran-overlay.nix
    (lnd.nix now uses pkgs.callPackage ../../packages/lndinit {})
- Remove pkgs/sovran-overlay.nix, pkgs/nbxplorer.nix, pkgs/README.md
  * No global overlay - lndinit is a normal package in packages/
- Remove modules/vendor/ entirely
- Update flake.nix: drop overlay-sovran, import ./modules/bitcoin
  instead of ./modules/vendor/nix-bitcoin/modules.nix
- No more random vendor/ or pkgs/ dirs - follows Sovran convention:
  modules/ for NixOS modules, packages/ for packages

modules/bitcoin/lib.nix

@@ -0,0 +1,132 @@
+lib: pkgs: config:
+
+with lib;
+
+# See `man systemd.exec` and `man systemd.resource-control` for an explanation
+# of the systemd-related options available through this file.
+let self = {
+  # These settings roughly follow systemd's "strict" security profile
+  defaultHardening = {
+      PrivateTmp = true;
+      ProtectSystem = "strict";
+      ProtectHome = true;
+      NoNewPrivileges = true;
+      PrivateDevices = true;
+      MemoryDenyWriteExecute = true;
+      ProtectKernelTunables = true;
+      ProtectKernelModules = true;
+      ProtectKernelLogs = true;
+      ProtectClock = true;
+      ProtectProc = "invisible";
+      ProcSubset = "pid";
+      ProtectControlGroups = true;
+      RestrictAddressFamilies = "AF_UNIX AF_INET AF_INET6";
+      RestrictNamespaces = true;
+      LockPersonality = true;
+      IPAddressDeny = "any";
+      PrivateUsers = true;
+      RestrictSUIDSGID = true;
+      RemoveIPC = true;
+      RestrictRealtime = true;
+      ProtectHostname = true;
+      CapabilityBoundingSet = "";
+      # @system-service whitelist and docker seccomp blacklist (except for "clone"
+      # which is a core requirement for systemd services)
+      # @system-service is defined in src/shared/seccomp-util.c (systemd source)
+      SystemCallFilter = [ "@system-service" "~add_key kcmp keyctl mbind move_pages name_to_handle_at personality process_vm_readv process_vm_writev request_key setns unshare userfaultfd" ];
+      SystemCallArchitectures = "native";
+  };
+
+  allowNetlink = {
+    RestrictAddressFamilies = self.defaultHardening.RestrictAddressFamilies + " AF_NETLINK";
+  };
+
+  nodejs = {
+    # Required for JIT compilation
+    MemoryDenyWriteExecute = false;
+    # Required by nodejs >= 18
+    SystemCallFilter = self.defaultHardening.SystemCallFilter ++ [ "@pkey" ];
+  };
+
+  # Allow takes precedence over Deny.
+  allowLocalIPAddresses = {
+    IPAddressAllow = [
+      "127.0.0.1/32"
+      "::1/128"
+      "169.254.0.0/16"
+    ];
+  };
+  allowAllIPAddresses = { IPAddressAllow = "any"; };
+  allowTor = self.allowLocalIPAddresses;
+  allowedIPAddresses = onlyLocal:
+    if onlyLocal
+    then self.allowLocalIPAddresses
+    else self.allowAllIPAddresses;
+
+  tor = {
+    proxy = mkOption {
+      type = types.bool;
+      default = false;
+      description = "Whether to proxy outgoing connections with Tor.";
+    };
+    enforce = mkOption {
+      type = types.bool;
+      default = false;
+      description = ''
+        Whether to enforce Tor on this service by only allowing connections
+        from and to localhost and link-local addresses.
+      '';
+    };
+  };
+
+  script = name: src: pkgs.writers.writeBash name ''
+    set -eo pipefail
+    ${src}
+  '';
+
+  # Used for ExecStart*
+  rootScript = name: src: "+${self.script name src}";
+
+  cliExec = mkOption {
+    # Used by netns-isolation to execute the cli in the service's private netns
+    internal = true;
+    type = types.str;
+    default = "exec";
+  };
+
+  mkOnionService = map: {
+    map = [ map ];
+    version = 3;
+  };
+
+  # Convert a bind address, which may be a special INADDR_ANY address,
+  # to an actual IP address
+  address = addr:
+    if addr == "0.0.0.0" then
+      "127.0.0.1"
+    else if addr == "::" then
+      "::1"
+    else
+      addr;
+
+  addressWithPort = addr: port: "${self.address addr}:${toString port}";
+
+  optionalAttr = cond: name: if cond then name else null;
+
+  mkCertExtraAltNames = cert:
+    builtins.concatStringsSep "," (
+      (map (domain: "DNS:${domain}") cert.extraDomains) ++
+      (map (ip: "IP:${ip}") cert.extraIPs)
+    );
+
+  test = {
+    mkIfTest = test: mkIf (config.tests.${test} or false);
+  };
+
+  mkAlias = default: mkOption {
+    internal = true;
+    readOnly = true;
+    inherit default;
+  };
+
+}; in self