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.

tests/test_bitcoin_tor_gossip.py

@@ -27,17 +27,12 @@ def _literal_assignment(source: str, name: str):
 
 
 class TestBitcoinTorGossipNixWiring(unittest.TestCase):
-    def test_bitcoind_loopback_listener_is_always_enabled(self):
-        ecosystem = _read("modules", "bitcoinecosystem.nix")
-        self.assertIn("listen = true;", ecosystem)
-        self.assertIn("peerbloomfilters=1", ecosystem)
-
-    def test_gossip_is_opt_in(self):
-        ecosystem = _read("modules", "bitcoinecosystem.nix")
-        self.assertIn(
-            "public = config.sovran_systemsOS.features.bitcoin-tor-gossip;",
-            ecosystem,
-        )
+    def test_integration_layer_maps_tor_gossip_option(self):
+        """The integration layer must bridge sovran_systemsOS.features.bitcoin-tor-gossip
+        to sovran-bitcoin.bitcoindTorGossip."""
+        integration = _read("modules", "sovran-bitcoin-integration.nix")
+        self.assertIn("bitcoindTorGossip", integration)
+        self.assertIn("bitcoin-tor-gossip", integration)
 
     def test_hub_option_and_evaluated_state_are_declared(self):
         roles = _read("modules", "core", "roles.nix")