feat(kernel): object lifetimes via slot generations

- SlotGenerations per table, never wraps, retires exhausted slots
- caps to dead/reused process, endpoint, region rejected
- endpoints owned, die w/ owner, waiters woken w/ error
- DropCap syscall + libkos wrapper
- irq table bounds-checked, routes dropped w/ thread
- self-kill terminates caller, exit_thread full cleanup
- sched run_now: switch to named thread, not queue head
- lifetime tests, fuzz kills/recreates, kani gen proofs

crates/domain/src/error.rs

@@ -31,4 +31,6 @@ pub enum KernelError {
     /// The target virtual range already holds a mapping.
     AddressInUse,
     InvalidRegion,
+    /// The object a capability would refer to no longer exists.
+    NoSuchObject,
 }