refactor: migrate away from deprecated SignOptions (signer module)

[darioAnongba]

Summary

BDK 2.2.0 deprecated SignOptions and the signer module in favor of bitcoin::psbt::Psbt::sign().

Currently bdk-wasm wraps SignOptions with #[allow(deprecated)] as a stopgap (PR #14). Need to investigate the replacement API and migrate Wallet::sign + SignOptions wrapper accordingly.

References

• bitcoin::Psbt::sign
• BDK 2.2.0 release notes
• Current workaround in src/bitcoin/wallet.rs

Acceptance Criteria

• Replace SignOptions usage with the new bitcoin::psbt signing API
• Remove #[allow(deprecated)] annotations
• Maintain backwards compatibility for JS consumers where possible
• All CI checks pass

[darioAnongba]

Investigation Notes

After reviewing BDK 2.3.0 source (wallet/src/wallet/mod.rs):

• The signer module is deprecated since 2.2.0 with note: "PSBT signing was moved to bitcoin::psbt module"
• However, Wallet::sign() itself is not deprecated and still requires SignOptions as a parameter
• Wallet::sign() internally uses the wallet's SignersContainer and calls signer.sign_transaction(psbt, &sign_options, &self.secp) for each signer
• There is no alternative signing method on Wallet that doesn't use SignOptions

Conclusion: This migration is currently blocked on upstream BDK. Until BDK provides a new Wallet::sign() API that doesn't depend on the deprecated SignOptions, we should keep the #[allow(deprecated)] annotations.

Recommendation: Keep this issue open and periodically check new BDK releases for the replacement API. The current workaround is correct and functional.