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/architecture/x86_64/idt.zig

@@ -3,9 +3,11 @@
 //! triple-faults and silently resets the machine. With it, the CPU vectors into
 //! our stubs, which capture the register state and hand it to a dispatcher.
 //!
-//! Vectors split in two: 0-31 are CPU exceptions (terminal — reported and
-//! halted); 32+ are device interrupts (a registered handler runs, the APIC is
-//! acknowledged, and we return to the interrupted code).
+//! Vectors split in two: 0-31 are CPU exceptions, handed to the `on_fault` hook
+//! and never returned from (the kernel's handler kills a faulting user process
+//! and reschedules, or halts the core for a kernel-mode fault); 32+ are device
+//! interrupts (a registered handler runs, the APIC is acknowledged, and we
+//! return to the interrupted code).
 
 const gdt = @import("gdt.zig");
 const tss = @import("tss.zig");
@@ -162,8 +164,9 @@ pub fn loadOnThisCpu() void {
 }
 
 /// Called by isr_common (isr.s) with a pointer to the trap frame. Exported so the
-/// assembly stubs can `call` it by name. Exceptions are terminal; device
-/// interrupts run their handler, get acknowledged, and return.
+/// assembly stubs can `call` it by name. Exceptions never return here (on_fault
+/// kills the faulting process or halts the core); device interrupts run their
+/// handler, get acknowledged, and return.
 export fn interruptDispatch(state: *CpuState) callconv(.c) void {
     if (state.vector < 32) {
         on_fault(state); // CPU exception — never returns