Kill a faulting user process instead of halting the machine

A CPU exception raised in ring 3 by a scheduled process now kills that
process - IRQ bindings, IPC handles, and address space reclaimed, a
client it owed a reply to failed with the new -EPEER instead of hung -
and the core reschedules (docs/resilience.md step 2). Kernel-mode
faults, NMI, double fault, and machine check stay terminal, as does the
borrowed-thread isolation probe. Proven by the new fault-recovery QEMU
test: init keeps heartbeating after a process page-faults to death.

system/kernel/ipc-synchronous.zig

@@ -46,6 +46,7 @@ pub const EFAULT: i64 = 3; // buffer unmapped / out of the user half
 pub const ENOENT: i64 = 4; // no such registered service
 pub const ENOSPC: i64 = 5; // handle table or registry full
 pub const ENOMEM: i64 = 6; // out of memory
+pub const EPEER: i64 = 7; // peer died before replying (its process exited or was killed)
 
 /// A badge with this bit set is an asynchronous notification (e.g. an IRQ), not a
 /// message from a client — there is no reply owed. The low bits carry the source