Add CI compatibility check against upstream bdk-ffi changes

[reez]

bdk-dart is downstream of bdk-ffi.
We should detect breakage early.

Scope:

• add a scheduled CI job
• build and test bdk-dart against latest bdk-ffi
• report failures clearly

Acceptance criteria:

• Scheduled workflow exists (daily or weekly).
• Workflow tests compatibility with upstream bdk-ffi.
• Failures are visible in CI logs.
• Requirements for production-ready release #16 links this issue.