fix: correct instruction index in calculate_size function

- Fixed incorrect get() index from total_size + 1 to proper instruction index
- Use enumerate() to get correct instruction index for prefetch optimization
- Prevents array bounds errors and ensures correct next instruction prefetch

Bug: Using total_size (byte count) as array index for instruction lookup
Solution: Use enumerate() iterator index for proper instruction access

src/trading/core/execution.rs

@@ -106,10 +106,10 @@ impl InstructionProcessor {
     pub fn calculate_size(instructions: &[Instruction]) -> usize {
         let mut total_size = 0;
 
-        for instr in instructions {
+        for (i, instr) in instructions.iter().enumerate() {
             // 预取下一条指令
             unsafe {
-                if let Some(next_instr) = instructions.get(total_size + 1) {
+                if let Some(next_instr) = instructions.get(i + 1) {
                     BranchOptimizer::prefetch_read_data(next_instr);
                 }
             }