feat(kernel): shared ABI decoder+proofs, no-panic gate, syscall storm, kani on shrunk kernel

crates/domain/src/message/mod.rs

@@ -31,8 +31,9 @@ impl Message {
         &self.words
     }
 
+    /// Word `index`, or 0 past the end.
     pub fn word(&self, index: usize) -> usize {
-        self.words[index]
+        self.words.get(index).copied().unwrap_or(0)
     }
 
     pub fn cap(&self) -> Option<CapSlot> {