Bridge canonicalize: add ArrayBox.len / MapBox.len diff tests; Stage‑B helpers: require v0 header; MapBox.get: add stable tag prefix [map/missing]; new core smokes (string lastIndexOf -1, map delete missing)

src/boxes/map_box.rs

@@ -154,7 +154,7 @@ impl MapBox {
                 }
                 value.clone_box()
             }
-            None => Box::new(StringBox::new(&format!("Key not found: {}", key_str))),
+            None => Box::new(StringBox::new(&format!("[map/missing] Key not found: {}", key_str))),
         }
     }