feat(parsers): add Curve StableSwap parser and fix ScaleToDecimals export

**Curve StableSwap Parser** (`curve.go`):
- TokenExchange event parsing (address,int128,uint256,int128,uint256)
- TokenExchangeUnderlying event support for wrapped tokens
- Coin index (int128) to token address mapping
- Handles 2-coin and multi-coin pools
- Typical use: USDC/USDT, DAI/USDC stablecoin swaps
- Low slippage due to amplification coefficient (A parameter)
- Fee: typically 0.04% (4 basis points)

**Key Features:**
- Buyer address extraction from indexed topics
- Coin ID to token mapping via pool cache
- Both directions: token0→token1 and token1→token0
- Buyer is both sender and recipient (Curve pattern)
- Support for 6-decimal stablecoins (USDC, USDT)

**Testing** (`curve_test.go`):
- TokenExchange and TokenExchangeUnderlying signature validation
- Swap direction tests (USDC→USDT, USDT→USDC)
- Multi-event receipts with mixed protocols
- Decimal scaling validation (6 decimals → 18 decimals)
- Pool not found error handling

**Type System Fix:**
- Exported ScaleToDecimals() function in pkg/types/pool.go
- Updated all callers to use exported function
- Fixed test function name (TestScaleToDecimals)
- Consistent across all parsers (V2, V3, Curve)

**Use Cases:**
1. Stablecoin arbitrage (Curve vs Uniswap pricing)
2. Low-slippage large swaps (Curve specialization)
3. Multi-coin pool support (3pool, 4pool)
4. Underlying vs wrapped token detection

**Task:** P2-018 (Curve StableSwap parser)
**Coverage:** 100% (enforced in CI/CD)
**Protocol:** Curve StableSwap on Arbitrum

🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>

pkg/types/pool.go

@@ -87,8 +87,8 @@ func (p *PoolInfo) CalculatePrice() *big.Float {
 	}
 
 	// Scale reserves to 18 decimals for consistent calculation
-	reserve0Scaled := scaleToDecimals(p.Reserve0, p.Token0Decimals, 18)
-	reserve1Scaled := scaleToDecimals(p.Reserve1, p.Token1Decimals, 18)
+	reserve0Scaled := ScaleToDecimals(p.Reserve0, p.Token0Decimals, 18)
+	reserve1Scaled := ScaleToDecimals(p.Reserve1, p.Token1Decimals, 18)
 
 	// Price = Reserve1 / Reserve0
 	reserve0Float := new(big.Float).SetInt(reserve0Scaled)
@@ -98,8 +98,8 @@ func (p *PoolInfo) CalculatePrice() *big.Float {
 	return price
 }
 
-// scaleToDecimals scales an amount from one decimal precision to another
-func scaleToDecimals(amount *big.Int, fromDecimals, toDecimals uint8) *big.Int {
+// ScaleToDecimals scales an amount from one decimal precision to another
+func ScaleToDecimals(amount *big.Int, fromDecimals, toDecimals uint8) *big.Int {
 	if fromDecimals == toDecimals {
 		return new(big.Int).Set(amount)
 	}

pkg/types/pool_test.go

@@ -237,7 +237,7 @@ func TestPoolInfo_CalculatePrice(t *testing.T) {
 	}
 }
 
-func Test_scaleToDecimals(t *testing.T) {
+func TestScaleToDecimals(t *testing.T) {
 	tests := []struct {
 		name         string
 		amount       *big.Int
@@ -277,9 +277,9 @@ func Test_scaleToDecimals(t *testing.T) {
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			got := scaleToDecimals(tt.amount, tt.fromDecimals, tt.toDecimals)
+			got := ScaleToDecimals(tt.amount, tt.fromDecimals, tt.toDecimals)
 			if got.Cmp(tt.want) != 0 {
-				t.Errorf("scaleToDecimals() = %v, want %v", got, tt.want)
+				t.Errorf("ScaleToDecimals() = %v, want %v", got, tt.want)
 			}
 		})
 	}