feat(vm): add call stack depth guard to MirInterpreter

src/backend/mir_interpreter/mod.rs

@@ -41,6 +41,9 @@ pub struct MirInterpreter {
     pub(super) inst_count: u64,
     pub(super) branch_count: u64,
     pub(super) compare_count: u64,
+    /// Call stack depth (exec_function_inner nesting). Used as a safety valve
+    /// to prevent Rust stack overflow on accidental infinite recursion in MIR.
+    pub(super) call_depth: usize,
 }
 
 impl MirInterpreter {
@@ -58,6 +61,7 @@ impl MirInterpreter {
             inst_count: 0,
             branch_count: 0,
             compare_count: 0,
+            call_depth: 0,
         }
     }