Span trace utilities and runner source hint

src/backend/mir_interpreter/mod.rs

@@ -33,6 +33,7 @@ pub struct MirInterpreter {
     // Trace context (dev-only; enabled with NYASH_VM_TRACE=1)
     pub(super) last_block: Option<BasicBlockId>,
     pub(super) last_inst: Option<MirInstruction>,
+    pub(super) last_inst_index: Option<usize>,
     // Static box singleton instances (persistent across method calls)
     pub(super) static_boxes: HashMap<String, crate::instance_v2::InstanceBox>,
     // Static box declarations (metadata for creating instances)
@@ -56,6 +57,7 @@ impl MirInterpreter {
             cur_fn: None,
             last_block: None,
             last_inst: None,
+            last_inst_index: None,
             static_boxes: HashMap::new(),
             static_box_decls: HashMap::new(),
             inst_count: 0,