test the trampoline is inert (zeroed + NX) when dormant

Adds paging.isExecutable and arch.trampolinePage; the smp case now asserts the frame is zeroed and non-executable after bring-up. Teeth-checked against a no-op disarm.

src/kernel/arch/x86_64/cpu.zig

@@ -130,6 +130,16 @@ pub fn testFailNextWakes(n: u32) void {
     smp.testFailNextWakes(n);
 }
 
+/// The reserved AP-trampoline frame (0 if none). For tests that check it's inert.
+pub fn trampolinePage() u64 {
+    return smp.trampolinePage();
+}
+
+/// Whether the page at `virt` is currently mapped executable (present, NX clear).
+pub fn pageExecutable(virt: u64) bool {
+    return paging.isExecutable(virt);
+}
+
 /// Kernel tick rate: 1000 Hz (1 ms), the scheduler's time quantum.
 pub const timer_hz = 1000;