fix(interrupts): use proper kernel stack when returning to idle loop

[ryanbreen]

Summary

• Fixes CI failure on QEMU 8.x (GitHub runners) that passes on QEMU 10.x (local)
• Root cause: returning to idle_loop using IST stack instead of proper kernel stack
• IST stacks are small (~4KB) and overflow when timer interrupts fire during idle_loop

Root Cause Analysis

When a userspace process faults and the page fault handler terminates it, the handler
sets up the exception frame to return to idle_loop. Previously, we used:

let current_rsp: u64;
core::arch::asm!("mov {}, rsp", out(reg) current_rsp);
frame.stack_pointer = x86_64::VirtAddr::new(current_rsp + 256);

This captures the CURRENT RSP which is on the IST page fault stack (IST[1]).
When IRET returns to idle_loop with this RSP, idle_loop runs on the small IST stack.
Timer interrupts then push frames onto this stack, eventually causing overflow and
corrupting RSP to values like 0xffffc97ffffffff0.

Fix

Use per_cpu::kernel_stack_top() to get the idle thread's actual kernel stack,
which is large (64KB) and meant for normal execution.

Test plan

• Local boot-stages test passes (138/138)
• CI boot-stages test passes

🤖 Generated with Claude Code