Expose Liquid invoice blinding secret

[BullishNode]

Summary

• pin lwk-dart to immutable commit 355dd786a634ae7dae8e55e82e9ba5f90a6ce6f5 from draft PR #91
• expose the generated addressWithBlindingSecret(index:) API through bull_sdk
• regenerate FRB bindings
• verify the per-address secret matches the address public blinding key

Status

Draft replacement for reverted PR #9. Do not merge until the mobile stack completes certification and the owner explicitly authorizes it.

Dependency stack

• base: Bull SDK draft PR chore: make Bull SDK own the BDK Dart binding #12
• LWK-Dart dependency: draft PR #91

This PR deliberately targets draft/reapply-bdk-dart, not main. Its feature code and generated bindings reproduce the previously reviewed change, with the immutable LWK pin moved to replacement draft PR #91.

Verification

• native integration test: passed on the original reviewed change
• focused Dart binding test: passed with Flutter 3.44.2 / Dart 3.12.2 on the original reviewed change
• Bull SDK release build: passed on the original reviewed change
• cargo check -p rust_lib_bull_sdk --locked: passed with the replacement LWK pin
• isolated FRB regeneration: no Dart drift; expected Rust post-processing remains required
• changed-file analysis and git diff --check: clean