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/io.rs

@@ -240,15 +240,18 @@ impl IoThread {
         self.outstanding += 1;
         // Send can only fail if the pool has hung up, which only happens
         // on shutdown. submit during shutdown is a bug.
-        self.tx
-            .send(Request { pid, epoch, work })
-            .expect("io pool hung up unexpectedly");
+        if self.tx.send(Request { pid, epoch, work }).is_err() {
+            panic!("smarm: io pool hung up unexpectedly (submit during shutdown)");
+        }
     }
 
     /// Drain every available completion. Caller (the scheduler) routes the
     /// results and updates `outstanding` / `waiters` accordingly.
     pub fn drain_completions(&mut self) -> Vec<Completion> {
-        let mut q = self.completions.lock().unwrap();
+        let mut q = match self.completions.lock() {
+            Ok(g) => g,
+            Err(e) => panic!("smarm: io completions lock poisoned (core corrupt): {e}"),
+        };
         let mut out = Vec::with_capacity(q.len());
         while let Some(c) = q.pop_front() {
             out.push(c);
@@ -389,10 +392,10 @@ fn pool_loop(
             Ok(r) => r,
             Err(payload) => Err(payload),
         };
-        completions
-            .lock()
-            .unwrap()
-            .push_back(Completion::Blocking { pid, epoch, result });
+        match completions.lock() {
+            Ok(mut g) => g.push_back(Completion::Blocking { pid, epoch, result }),
+            Err(e) => panic!("smarm: io completions lock poisoned (core corrupt): {e}"),
+        }
         wake_scheduler(wake_write);
     }
 }
@@ -435,7 +438,10 @@ fn epoll_loop(
         let mut shutdown_requested = false;
         let mut pushed_any = false;
         {
-            let mut q = completions.lock().unwrap();
+            let mut q = match completions.lock() {
+                Ok(g) => g,
+                Err(e) => panic!("smarm: io completions lock poisoned (core corrupt): {e}"),
+            };
             for ev in events.iter().take(n as usize) {
                 if ev.u64 == SHUTDOWN_EPOLL_TOKEN {
                     shutdown_requested = true;