Bridge canonicalize: add PhiInst.lower_phi and LLVMConstInstructionBox.lower_const diff tests; ArrayBox.pop empty strict tag [array/empty/pop] + smoke; VM README: document [map/missing] and [array/empty/pop] tags

src/boxes/array/mod.rs

@@ -39,7 +39,21 @@ impl ArrayBox {
     pub fn pop(&self) -> Box<dyn NyashBox> {
         match self.items.write().unwrap().pop() {
             Some(item) => item,
-            None => Box::new(crate::boxes::null_box::NullBox::new()),
+            None => {
+                let strict = std::env::var("HAKO_OOB_STRICT")
+                    .ok()
+                    .map(|v| matches!(v.as_str(), "1"|"true"|"on"))
+                    .unwrap_or(false)
+                    || std::env::var("NYASH_OOB_STRICT")
+                        .ok()
+                        .map(|v| matches!(v.as_str(), "1"|"true"|"on"))
+                        .unwrap_or(false);
+                if strict {
+                    Box::new(StringBox::new("[array/empty/pop] empty array"))
+                } else {
+                    Box::new(crate::boxes::null_box::NullBox::new())
+                }
+            }
         }
     }