feat(pkc): expose BLS secret-share recovery #113

Summary

Expose BLS secret-share recovery in dash-pkc, reusing the threshold ID validation and interpolation machinery already owned by the crypto crate.

Caller

Platform PR #5323 removes dashpay/bls-signatures. Its mock quorum builder needs to recover a secret key from participant secret shares. BlsPublicKey::recover_shares and signature recovery are available, but the pinned base-sdk revision addedfa16a56c77f435b41804cd2c0668b776e97 has no equivalent secret-key recovery API. The same API is absent on develop at 02ba9c27e51826d0d8a4d6353b9f7c9422389f5b.

Platform currently carries a mock-only scalar implementation through dash_pkc::__deps::ff. Moving this responsibility into dash-pkc lets Platform delegate through its BLS adapter and remove the local interpolation implementation.

Required behavior

• Recover a BLS secret key from paired participant IDs and secret shares, interpolating at zero.
• Reuse the existing reduced-ID validation: interpret 32-byte IDs as big endian modulo the scalar order, reject zero-reducing IDs and IDs equal modulo that order.
• Reject fewer than two shares and a zero recovered secret with typed errors.
• Match existing Core-compatible secret/public key bytes and Basic signatures. Platform has independent frozen vectors for 2, 3 and 5 members in core_quorums.json.

Implementation belongs in pkgs/pkc/src/bls/share_ops.rs / scheme_ops.rs, beside public-key and signature recovery. After publishing the API, update rust-dashcore/Platform adapters and replace Platform's mock helper.

_{🤖 Co-authored by Claudius the Magnificent AI Agent}