fix: add stubs for nixpkgs-unstable 2026-08 where services.clightning removed

nixpkgs 8b8c811 (2026-08-08) removed services.clightning.enable,
causing btcpayserver.nix:124 to throw 'option does not exist' on
nixos-rebuild (your error). Sovran uses lnd only, never clightning,
but evaluation still throws.

- Add modules/vendor/nix-bitcoin/stubs.nix to provide missing options
  as false stubs: clightning, clightning-rest, liquidd, fulcrum,
  lightning-loop/pool, joinmarket
- Guard btcpayserver/rtl/lnd/mempool clightning/liquidd references
  with config.services ? X checks
- Trim enable-tor.nix onionServices for removed services

modules/vendor/nix-bitcoin/modules.nix

@@ -3,8 +3,10 @@
 # Only services actually used by Sovran are kept (6 services vs 20+ upstream)
 # - backups.nix removed: Sovran uses rsnapshot to Second_Drive (configuration.nix)
 # - netns-isolation.nix is now a stub (requires false for nwc-wallets)
+# - stubs.nix provides options for services referenced but not in nixpkgs unstable 2026-08
 {
   imports = [
+    ./stubs.nix
     ./nix-bitcoin.nix
     ./secrets/secrets.nix
     ./operator.nix