refactor: extract bitcoin stack into Sovran_Bitcoin flake input

Decouple the Bitcoin/Lightning modules and packages into the standalone
Sovran_Bitcoin flake, consumed as a NixOS module input.

Deleted (now in Sovran_Bitcoin):
  - modules/bitcoin/          (19 files — vendored nix-bitcoin modules)
  - modules/bitcoinecosystem.nix
  - modules/nwc-wallets.nix
  - modules/mempool.nix
  - packages/{albyhub,mempool,rtl,build-support}/
  - tests/bitcoin-btcpay-hardening.nix

Created:
  - modules/sovran-bitcoin-integration.nix — the OS-specific bridge that
    maps sovran_systemsOS.* options to sovran-bitcoin.* and applies
    Second_Drive paths, operator 'free', forced wallet, firewall 3051,
    and Sovran Hub NWC environment wiring.

Modified:
  - flake.nix — added sovran-bitcoin flake input, updated module imports
  - modules/modules.nix — removed deleted imports
  - modules/core/sovran-hub.nix — version metadata now reads from
    pkgs.sovran-bitcoin.* overlay instead of local packages/
  - tests/test_bitcoin_tor_gossip.py — updated to check integration layer

The sovran_systemsOS.* option namespace is preserved. The Hub, roles,
and custom.nix continue to work unchanged.

modules/core/role-logic.nix

@@ -3,7 +3,7 @@
 {
   config = lib.mkMerge [
 
-    # Vendored nix-bitcoin is always imported via modules/vendor/nix-bitcoin/modules.nix.
+    # The Sovran_Bitcoin flake module is always imported (see flake.nix).
     # This default satisfies the secrets assertion so Desktop-Only systems evaluate
     # without enabling any Bitcoin services.
     {