feat: add comprehensive transaction building to wallet FFI

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](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: PastaPastaPasta

key-wallet-ffi/include/key_wallet_ffi.h

@@ -3182,7 +3182,10 @@ bool mnemonic_to_seed(const char *mnemonic,
  void mnemonic_free(char *mnemonic) ;
 
 /*
- Build a transaction
+ Build a transaction (unsigned)
+
+ This creates an unsigned transaction. Use wallet_sign_transaction to sign it afterward.
+ For a combined build+sign operation, use wallet_build_and_sign_transaction.
 
  # Safety
 
@@ -3227,6 +3230,40 @@ bool wallet_sign_transaction(const FFIWallet *wallet,
                              FFIError *error)
 ;
 
+/*
+ Build and sign a transaction using the wallet's managed info
+
+ This is the recommended way to build transactions. It handles:
+ - UTXO selection using coin selection algorithms
+ - Fee calculation
+ - Change address generation
+ - Transaction signing
+
+ # Safety
+
+ - `managed_wallet` must be a valid pointer to an FFIManagedWalletInfo
+ - `wallet` must be a valid pointer to an FFIWallet
+ - `network` must be a valid FFINetwork
+ - `outputs` must be a valid pointer to an array of FFITxOutput with at least `outputs_count` elements
+ - `tx_bytes_out` must be a valid pointer to store the transaction bytes pointer
+ - `tx_len_out` must be a valid pointer to store the transaction length
+ - `error` must be a valid pointer to an FFIError
+ - The returned transaction bytes must be freed with `transaction_bytes_free`
+ */
+
+bool wallet_build_and_sign_transaction(FFIManagedWalletInfo *managed_wallet,
+                                       const FFIWallet *wallet,
+                                       FFINetwork network,
+                                       unsigned int account_index,
+                                       const FFITxOutput *outputs,
+                                       size_t outputs_count,
+                                       uint64_t fee_per_kb,
+                                       uint32_t current_height,
+                                       uint8_t **tx_bytes_out,
+                                       size_t *tx_len_out,
+                                       FFIError *error)
+;
+
 /*
  Check if a transaction belongs to the wallet using ManagedWalletInfo
 
@@ -3313,6 +3350,20 @@ bool wallet_check_transaction(FFIWallet *wallet,
  */
  int32_t transaction_get_txid(const FFITransaction *tx, uint8_t *txid_out) ;
 
+/*
+ Get transaction ID from raw transaction bytes
+
+ # Safety
+ - `tx_bytes` must be a valid pointer to transaction bytes
+ - `tx_len` must be the correct length of the transaction
+ - `error` must be a valid pointer to an FFIError
+
+ # Returns
+ - Pointer to null-terminated hex string of TXID (must be freed with string_free)
+ - NULL on error
+ */
+ char *transaction_get_txid_from_bytes(const uint8_t *tx_bytes, size_t tx_len, FFIError *error) ;
+
 /*
  Serialize a transaction