Use current instruction for f_lineno addr2line

lasti is stored as the next instruction index. addr2line of
that byte offset is past RETURN_VALUE, so f_lineno was None
and stdlib_threading_current_frames hung on leftover workers.

Assisted-by: Grok:grok-4.6

Author: youknowone

crates/vm/src/builtins/frame.rs

@@ -525,7 +525,9 @@ impl FrameObject {
                 return prev as i32;
             }
         }
-        self.f_code().addr2line(lasti_bytes)
+        // lasti is stored as the next instruction index (see FrameObject::run),
+        // so the executing opcode is at lasti-1 / lasti_bytes-2.
+        self.f_code().addr2line(lasti_bytes - 2)
     }
 
     #[pygetset(name = "f_lineno")]