core: rewrite panic sites as explicit match+panic

Replace implicit unwrap()/expect() in the lock-ordered core with explicit
match arms. Lock-poison sites use one uniform message
("smarm: <lock> lock poisoned (core corrupt): {e}"); invariant sites panic
with a descriptive message naming the violated invariant. No behaviour
change: each rewrite preserves the prior panic-on-bad-arm semantics. Also
clears the accompanying clippy hygiene in these files (redundant_closure,
len_without_is_empty, too_many_arguments, unnecessary_sort_by,
missing_safety_doc, nonminimal_bool/unnecessary_unwrap).

src/registry.rs

@@ -193,11 +193,12 @@ impl Mailbox {
     /// legal under a Leaf (Leaf -> Channel).
     fn clone_sender<M: Send + 'static>(&self) -> Option<Sender<M>> {
         let ch = self.channels.get(&TypeId::of::<M>())?;
-        let tx = ch
-            .sender
-            .as_any()
-            .downcast_ref::<Sender<M>>()
-            .expect("channel keyed by TypeId but downcast to its own type failed — smarm bug");
+        let tx = match ch.sender.as_any().downcast_ref::<Sender<M>>() {
+            Some(tx) => tx,
+            None => panic!(
+                "smarm: channel keyed by TypeId but downcast to its own type failed (core corrupt)"
+            ),
+        };
         debug_assert_eq!(ch.msg_type, type_name::<M>(), "msg_type / TypeId disagree");
         Some(tx.clone())
     }