feat(consensus): miner transaction validation

ValidateMinerTx checks genesis input height, input count (1 for PoW,
2 for PoS), and stake input type per hardfork version.

Co-Authored-By: Charon <charon@lethean.io>

consensus/block.go

@@ -10,6 +10,7 @@ import (
 	"sort"
 
 	"forge.lthn.ai/core/go-blockchain/config"
+	"forge.lthn.ai/core/go-blockchain/types"
 )
 
 // IsPoS returns true if the block flags indicate a Proof-of-Stake block.
@@ -57,3 +58,42 @@ func medianTimestamp(timestamps []uint64) uint64 {
 	}
 	return sorted[n/2]
 }
+
+// ValidateMinerTx checks the structure of a coinbase (miner) transaction.
+// For PoW blocks: exactly 1 input (TxInputGenesis). For PoS blocks: exactly
+// 2 inputs (TxInputGenesis + stake input).
+func ValidateMinerTx(tx *types.Transaction, height uint64, forks []config.HardFork) error {
+	if len(tx.Vin) == 0 {
+		return fmt.Errorf("%w: no inputs", ErrMinerTxInputs)
+	}
+
+	// First input must be TxInputGenesis.
+	gen, ok := tx.Vin[0].(types.TxInputGenesis)
+	if !ok {
+		return fmt.Errorf("%w: first input is not txin_gen", ErrMinerTxInputs)
+	}
+	if gen.Height != height {
+		return fmt.Errorf("%w: got %d, expected %d", ErrMinerTxHeight, gen.Height, height)
+	}
+
+	// PoW blocks: exactly 1 input. PoS: exactly 2.
+	if len(tx.Vin) == 1 {
+		// PoW — valid.
+	} else if len(tx.Vin) == 2 {
+		// PoS — second input must be a spend input.
+		switch tx.Vin[1].(type) {
+		case types.TxInputToKey:
+			// Pre-HF4 PoS.
+		default:
+			hf4Active := config.IsHardForkActive(forks, config.HF4Zarcanum, height)
+			if !hf4Active {
+				return fmt.Errorf("%w: invalid PoS stake input type", ErrMinerTxInputs)
+			}
+			// Post-HF4: accept ZC inputs.
+		}
+	} else {
+		return fmt.Errorf("%w: %d inputs (expected 1 or 2)", ErrMinerTxInputs, len(tx.Vin))
+	}
+
+	return nil
+}

consensus/block_test.go

@@ -7,6 +7,7 @@ import (
 	"time"
 
 	"forge.lthn.ai/core/go-blockchain/config"
+	"forge.lthn.ai/core/go-blockchain/types"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
@@ -62,3 +63,56 @@ func TestCheckTimestamp_Ugly(t *testing.T) {
 	err := CheckTimestamp(now-200, 0, now, timestamps) // old but under 60 entries
 	require.NoError(t, err)
 }
+
+func validMinerTx(height uint64) *types.Transaction {
+	return &types.Transaction{
+		Version: types.VersionInitial,
+		Vin:     []types.TxInput{types.TxInputGenesis{Height: height}},
+		Vout: []types.TxOutput{
+			types.TxOutputBare{Amount: config.BlockReward, Target: types.TxOutToKey{Key: types.PublicKey{1}}},
+		},
+	}
+}
+
+func TestValidateMinerTx_Good(t *testing.T) {
+	tx := validMinerTx(100)
+	err := ValidateMinerTx(tx, 100, config.MainnetForks)
+	require.NoError(t, err)
+}
+
+func TestValidateMinerTx_Bad_WrongHeight(t *testing.T) {
+	tx := validMinerTx(100)
+	err := ValidateMinerTx(tx, 200, config.MainnetForks) // height mismatch
+	assert.ErrorIs(t, err, ErrMinerTxHeight)
+}
+
+func TestValidateMinerTx_Bad_NoInputs(t *testing.T) {
+	tx := &types.Transaction{Version: types.VersionInitial}
+	err := ValidateMinerTx(tx, 100, config.MainnetForks)
+	assert.ErrorIs(t, err, ErrMinerTxInputs)
+}
+
+func TestValidateMinerTx_Bad_WrongFirstInput(t *testing.T) {
+	tx := &types.Transaction{
+		Version: types.VersionInitial,
+		Vin:     []types.TxInput{types.TxInputToKey{Amount: 1}},
+	}
+	err := ValidateMinerTx(tx, 100, config.MainnetForks)
+	assert.ErrorIs(t, err, ErrMinerTxInputs)
+}
+
+func TestValidateMinerTx_Good_PoS(t *testing.T) {
+	tx := &types.Transaction{
+		Version: types.VersionInitial,
+		Vin: []types.TxInput{
+			types.TxInputGenesis{Height: 100},
+			types.TxInputToKey{Amount: 1}, // PoS stake input
+		},
+		Vout: []types.TxOutput{
+			types.TxOutputBare{Amount: config.BlockReward, Target: types.TxOutToKey{Key: types.PublicKey{1}}},
+		},
+	}
+	err := ValidateMinerTx(tx, 100, config.MainnetForks)
+	// 2 inputs with genesis + TxInputToKey is valid PoS structure.
+	require.NoError(t, err)
+}