add a big kernel lock for SMP

Guards the scheduler and IPC; held across the context switch.

src/kernel/arch/x86_64/cpu.zig

@@ -282,3 +282,11 @@ pub fn readCr2() u64 {
 pub fn halt() noreturn {
     while (true) asm volatile ("hlt");
 }
+
+/// Spin-wait hint (`pause`). Emitted in the body of a spinlock's busy-wait: it
+/// relaxes the core while it polls a contended lock — yielding pipeline resources
+/// to a hyperthread sibling and easing the cache-coherency traffic on the lock
+/// line. Purely a performance/power hint; correct to omit, but kinder on the bus.
+pub fn cpuRelax() void {
+    asm volatile ("pause");
+}

src/kernel/arch/x86_64/isr.s

@@ -63,9 +63,14 @@ switch_context:
     ret                     # return into the new task's saved instruction pointer
 
 # task_trampoline: the first thing a freshly-spawned task runs. init_task_stack
-# leaves its entry function in r15. New tasks start with interrupts enabled.
+# leaves its entry function in r15. A fresh task is switched to with the big kernel
+# lock held (the hand-off rule in sync.zig) but has no enter/leave frame of its own,
+# so it releases the lock here before running its body. r15 survives the call (it's
+# callee-saved). New tasks then start with interrupts enabled.
+.extern releaseForFreshTask
 .global task_trampoline
 task_trampoline:
+    call releaseForFreshTask # drop the kernel lock we inherited across the switch
     sti
     call *%r15              # call the task entry (fn() void)
 1:  hlt                     # if the entry returns, idle (still preemptible)