refactor: VM module - split execute_instruction into modular handlers

- Extracted 29 instruction handlers from 691-line execute_instruction function
- Created vm_instructions.rs module for better code organization
- Deleted legacy execute_instruction_old function (691 lines removed)
- Reduced vm.rs from 2075 to 1382 lines (33% reduction)

Benefits:
- Each instruction handler is now independently testable
- Improved maintainability and readability
- Easier to add new VM instructions
- Better separation of concerns

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/backend/mod.rs

@@ -4,6 +4,7 @@
 
 pub mod vm;
 pub mod vm_phi;
+pub mod vm_instructions;
 
 #[cfg(feature = "wasm-backend")]
 pub mod wasm;