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.

THIRD_PARTY_NOTICES.md

@@ -2,10 +2,13 @@
 
 ## nix-bitcoin
 
-Portions of `modules/bitcoin/` and selected package definitions were adapted
-from [fort-nix/nix-bitcoin](https://github.com/fort-nix/nix-bitcoin), commit
+The Bitcoin and Lightning modules were originally adapted from
+[fort-nix/nix-bitcoin](https://github.com/fort-nix/nix-bitcoin), commit
 [`360e30fee5ba32f9fecc89bc35628195d9d2dbbe`](https://github.com/fort-nix/nix-bitcoin/commit/360e30fee5ba32f9fecc89bc35628195d9d2dbbe).
-The current Sovran build does not import or fetch nix-bitcoin.
+They now live in the standalone
+[Sovran_Bitcoin](https://github.com/naturallaw777/Sovran_Bitcoin) repository
+and are consumed by Sovran_SystemsOS as a flake input. The current Sovran build
+does not import or fetch nix-bitcoin.
 
 MIT License