feat: add comprehensive transaction building to wallet FFI

[PastaPastaPasta]

This commit adds production-ready transaction building functionality to the key-wallet-ffi layer, enabling C/Swift clients to create and sign transactions.

New FFI Functions:

1. wallet_build_and_sign_transaction() - Comprehensive transaction builder that:

   • Selects UTXOs using Branch and Bound coin selection algorithm
   • Calculates fees based on fee_per_kb rate
   • Generates change addresses from internal address pool
   • Derives private keys from root extended key for signing
   • Returns fully signed, serialized transaction bytes

2. transaction_get_txid_from_bytes() - Utility function that:

   • Deserializes raw transaction bytes
   • Returns hex-encoded TXID as C string

Implementation Details:

• Integrates with ManagedWalletInfo for UTXO tracking and address management
• Uses TransactionBuilder pattern with proper fee calculation
• Maps addresses to derivation paths for key derivation
• Comprehensive error handling with FFIError propagation
• Proper memory management following FFI safety patterns
• Updated wallet_build_transaction() to direct users to new combined function

🤖 Generated with Claude Code

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📥 Commits

Reviewing files that changed from the base of the PR and between 35e47fd and 65b66e8.

📒 Files selected for processing (3)

• key-wallet-ffi/FFI_API.md (7 hunks)
• key-wallet-ffi/include/key_wallet_ffi.h (3 hunks)
• key-wallet-ffi/src/transaction.rs (6 hunks)

✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch feat/wallet-ffi-transaction-builder

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[github-actions]

⚠️ FFI Documentation Update Required

The FFI API documentation is out of date. Please regenerate it by running:

For key-wallet-ffi:

cd key-wallet-ffi
make update-docs

For dash-spv-ffi:

cd dash-spv-ffi
make update-docs

Then commit the changes:

git add */FFI_API.md
git commit -m "docs: update FFI API documentation"

This ensures the documentation stays in sync with the actual FFI functions.

[PastaPastaPasta]

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