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/supervisor.rs

@@ -253,7 +253,7 @@ impl OneForOne {
                     .collect(),
             };
             // Stop survivors in reverse start order (highest child index first).
-            to_stop.sort_unstable_by(|a, b| b.1.cmp(&a.1));
+            to_stop.sort_unstable_by_key(|x| std::cmp::Reverse(x.1));
 
             // The set we will restart: the failed child plus every sibling we
             // are about to stop, restarted in start (ascending index) order.
@@ -299,7 +299,7 @@ impl OneForOne {
         // and this is a no-op; on a cap-trip or mailbox-closed break it tears
         // the remaining children down deterministically instead of leaking them.
         let mut survivors: Vec<(Pid, usize)> = by_pid.iter().map(|(p, i)| (*p, *i)).collect();
-        survivors.sort_unstable_by(|a, b| b.1.cmp(&a.1));
+        survivors.sort_unstable_by_key(|x| std::cmp::Reverse(x.1));
         let mut awaiting: Vec<Pid> = Vec::with_capacity(survivors.len());
         for (pid, _) in &survivors {
             crate::scheduler::request_stop(*pid);