WIP: Add Store instruction to MIR for variable assignments

- Added Store instruction generation in build_assignment()
- This partially addresses the VM infinite loop issue
- However, the loop still uses old values (%0) instead of updated values
- Need to implement proper SSA phi nodes for loop variables

The root cause: MIR generation doesn't properly track variable updates
in loops. Current SSA implementation lacks phi nodes.

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

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

src/mir/builder.rs

@@ -316,7 +316,15 @@ impl MirBuilder {
         let value_id = self.build_expression(value)?;
         
         // In SSA form, each assignment creates a new value
-        self.variable_map.insert(var_name, value_id);
+        self.variable_map.insert(var_name.clone(), value_id);
+        
+        // Generate a Store instruction to ensure VM can track the assignment
+        // For now, we use the variable name as a simple pointer identifier
+        let var_ptr = self.value_gen.next();
+        self.emit_instruction(MirInstruction::Store {
+            value: value_id,
+            ptr: var_ptr,
+        })?;
         
         Ok(value_id)
     }