api: Stop exposing Bitcoin Core descriptor records #63

Follow-up to the review question on #12 and the v1 API-boundary pass.

core_descriptors(...) is a legacy public adapter from before Bitcoin Core became the sole EC/HD-key integration. The required v1 cleanup is:

• remove core_descriptors from package-level codex32.__all__ and supported public documentation;
• remove the unused public-deriver abstraction/branch from wallet.py;
• keep descriptor/checksum helpers still needed by the Core adapter/tests/tools private;
• preserve Wallet: use Core for setup and remove test crypto deps #7's Core-native account-0 initialization and do not reintroduce the obsolete pre-Wallet: use Core for setup and remove test crypto deps #7 import-record path;
• leave nonzero Core-native account support to wallet: Support nonzero accounts with Core-native setup #68;
• keep master_xprv unchanged;
• rerun public-API/Core tests and the real-Core fixture.

Focused PR #130 supersedes #64. It removes the obsolete descriptor API plus the likewise-unused fresh Core Lightning generation exports, leaving the package-level codex32.__all__ at 22 names while preserving parsing/recovery/re-sharing of existing CL secrets. Current head 2cbd279 has a green Python matrix and real-Core fixture, its one Codex documentation finding is resolved, and Codex reports no major issue on that exact head.

No automated/code-review gap remains. Keep this issue open through responsible-human review and integration of #130.