fix: harden kernel — eliminate UB, proper process cleanup

Critical: kernel_ref() UB eliminated
- kernel_ref() returned &'static mut, creating aliasing mutable
  references across trap handler + boot code (13+ call sites)
- Replaced with kernel_ptr() -> *mut Kernel, each use site does
  explicit unsafe { &mut *kernel_ptr() }
- KernelCell: removed get_mut(), added as_ptr() -> *mut T
- UB is gone: raw pointer access makes the single-core invariant
  explicit at every use site

Important: process cleanup on exit/fault
- New destroy_process() method consolidates cleanup:
  - Removes all threads from scheduler and endpoints
  - Clears pending messages and results
  - Destroys process and frees ProcessMemory
- handle_exit and handle_fault both use destroy_process()
- handle_exit_thread uses destroy_process when last thread exits

IPC hardening:
- Added invariant comment on pending_messages[sender].take()
  (sender in wait queue must have a pending message)

handle_release_pages:
- Now uses translate_va to convert VA→PA (was treating VA as PA)

crates/kernel/src/syscall/fault.rs

@@ -14,6 +14,7 @@ impl<P: Platform> Kernel<P> {
             Err(_) => return,
         };
 
+        // Try to deliver fault to a fault handler endpoint (if configured)
         if let Ok(process) = self.processes.get(pid) {
             if let Some(handler_slot) = process.fault_handler() {
                 if let Ok(cap) = process.capabilities().get(handler_slot) {
@@ -46,21 +47,6 @@ impl<P: Platform> Kernel<P> {
             }
         }
 
-        let thread_ids: [Option<types::id::ThreadId>; 4] = {
-            match self.processes.get(pid) {
-                Ok(p) => {
-                    let mut ids = [None; 4];
-                    for (i, t) in p.thread_ids().enumerate() {
-                        if i < 4 { ids[i] = Some(t); }
-                    }
-                    ids
-                }
-                Err(_) => return,
-            }
-        };
-
-        self.cleanup_threads(&thread_ids);
-        let _ = self.processes.destroy(pid);
-        self.process_memory[pid.raw() as usize] = None;
+        self.destroy_process(pid);
     }
 }