feat(wallet): transaction builder, UTXO selection, and status poller (Task 4)

## What's new

### API module additions
- ProtocolParameters: data class for fee calculation params
- PaymentRequest: transaction request model
- SignedTransaction: signed transaction result model
- TransactionBuilder: interface for building/signing transactions
- PaymentStatusPoller: interface for polling tx confirmation

### CardanoClient updates
- Added getProtocolParameters() to interface
- Implemented in KoiosCardanoClient with retry logic

### Implementation
- DefaultTransactionBuilder: builds and signs transactions using cardano-client-lib
  - Largest-first UTXO selection
  - Fee calculation via protocol parameters
  - Min UTXO validation (1 ADA minimum)
  - Secure key handling (zeroed after use)
- DefaultPaymentStatusPoller: polls Koios for tx confirmation
  - 10s polling interval, 60 attempts max (~10 minutes)
  - Emits TxStatus.PENDING -> CONFIRMED/FAILED flow

### Test module
- FakeTransactionBuilder: configurable success/failure responses
- FakePaymentStatusPoller: configurable status sequences
- Updated FakeCardanoClient with getProtocolParameters()

### Unit tests
- TransactionBuilderTest: UTXO selection, fee calculation, error handling
- PaymentStatusPollerTest: polling behavior, error recovery

features/wallet/api/src/main/kotlin/io/element/android/features/wallet/api/CardanoClient.kt

@@ -44,4 +44,13 @@ interface CardanoClient {
      * @return Current [TxStatus] of the transaction
      */
     suspend fun getTxStatus(txHash: String): Result<TxStatus>
+
+    /**
+     * Get the current protocol parameters from the network.
+     *
+     * Protocol parameters are needed for fee calculation and transaction building.
+     *
+     * @return Current [ProtocolParameters] from the latest epoch
+     */
+    suspend fun getProtocolParameters(): Result<ProtocolParameters>
 }

features/wallet/api/src/main/kotlin/io/element/android/features/wallet/api/PaymentRequest.kt

@@ -0,0 +1,24 @@
+/*
+ * Copyright (c) 2026 Sulkta Coop.
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package io.element.android.features.wallet.api
+
+import io.element.android.libraries.matrix.api.core.SessionId
+
+/**
+ * A request to build and sign a Cardano payment transaction.
+ *
+ * @property fromAddress The sender's Cardano address (Bech32)
+ * @property toAddress The recipient's Cardano address (Bech32)
+ * @property amountLovelace The amount to send in lovelace (1 ADA = 1,000,000 lovelace)
+ * @property sessionId The Matrix session ID for key retrieval
+ */
+data class PaymentRequest(
+    val fromAddress: String,
+    val toAddress: String,
+    val amountLovelace: Long,
+    val sessionId: SessionId,
+)

features/wallet/api/src/main/kotlin/io/element/android/features/wallet/api/PaymentStatusPoller.kt

@@ -0,0 +1,32 @@
+/*
+ * Copyright (c) 2026 Sulkta Coop.
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package io.element.android.features.wallet.api
+
+import kotlinx.coroutines.flow.Flow
+
+/**
+ * Interface for polling transaction confirmation status.
+ */
+interface PaymentStatusPoller {
+    /**
+     * Polls for transaction confirmation status.
+     *
+     * Emits [TxStatus] changes as a Flow:
+     * - Initially PENDING
+     * - CONFIRMED when transaction is in a block
+     * - FAILED if confirmation times out or error occurs
+     *
+     * Polling behavior:
+     * - Poll every 10 seconds
+     * - Maximum 60 attempts (~10 minutes total)
+     * - Stops when status changes from PENDING
+     *
+     * @param txHash The transaction hash to poll
+     * @return Flow of [TxStatus] changes
+     */
+    fun pollUntilConfirmed(txHash: String): Flow<TxStatus>
+}

features/wallet/api/src/main/kotlin/io/element/android/features/wallet/api/ProtocolParameters.kt

@@ -0,0 +1,25 @@
+/*
+ * Copyright (c) 2026 Sulkta Coop.
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package io.element.android.features.wallet.api
+
+/**
+ * Cardano protocol parameters needed for transaction building and fee calculation.
+ *
+ * These parameters are set via governance and determine transaction costs
+ * and constraints on the network.
+ *
+ * @property minFeeA The linear fee coefficient (lovelace per byte)
+ * @property minFeeB The constant fee (base fee in lovelace)
+ * @property maxTxSize Maximum transaction size in bytes
+ * @property utxoCostPerByte Cost in lovelace per byte of UTXO storage (for min UTXO calculation)
+ */
+data class ProtocolParameters(
+    val minFeeA: Long,
+    val minFeeB: Long,
+    val maxTxSize: Int,
+    val utxoCostPerByte: Long,
+)

features/wallet/api/src/main/kotlin/io/element/android/features/wallet/api/SignedTransaction.kt

@@ -0,0 +1,22 @@
+/*
+ * Copyright (c) 2026 Sulkta Coop.
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package io.element.android.features.wallet.api
+
+/**
+ * A signed Cardano transaction ready for submission.
+ *
+ * @property txCbor The CBOR-encoded signed transaction as a hex string
+ * @property txHash The transaction hash (for tracking)
+ * @property fee The transaction fee in lovelace
+ * @property actualAmount The actual amount sent (may differ slightly from requested due to min UTXO rules)
+ */
+data class SignedTransaction(
+    val txCbor: String,
+    val txHash: String,
+    val fee: Long,
+    val actualAmount: Long,
+)

features/wallet/api/src/main/kotlin/io/element/android/features/wallet/api/TransactionBuilder.kt

@@ -0,0 +1,40 @@
+/*
+ * Copyright (c) 2026 Sulkta Coop.
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package io.element.android.features.wallet.api
+
+/**
+ * Interface for building and signing Cardano transactions.
+ *
+ * The implementation handles:
+ * - UTXO selection (largest-first coin selection)
+ * - Fee calculation based on protocol parameters
+ * - Change output calculation
+ * - Transaction signing with the spending key
+ *
+ * ## Error handling
+ * The following errors may be returned:
+ * - [CardanoException.InsufficientFundsException] - Not enough ADA in wallet
+ * - [CardanoException.InvalidAddressException] - Invalid address format
+ * - [CardanoException.ApiException] - Various API/build errors
+ */
+interface TransactionBuilder {
+    /**
+     * Builds and signs a payment transaction.
+     *
+     * This method will:
+     * 1. Fetch UTXOs for the sender address
+     * 2. Select UTXOs to cover amount + fee (largest-first)
+     * 3. Build the transaction with proper change output
+     * 4. Retrieve the spending key (triggers biometric prompt)
+     * 5. Sign the transaction
+     * 6. Return the signed transaction ready for submission
+     *
+     * @param request The payment request details
+     * @return [SignedTransaction] on success, error on failure
+     */
+    suspend fun buildAndSign(request: PaymentRequest): Result<SignedTransaction>
+}