add smp-retry test for the AP wake retry path

Test hook forces the first wake to fail; the case asserts every core still comes online. Verified it fails when retry is disabled.

src/kernel/arch/x86_64/cpu.zig

@@ -124,6 +124,12 @@ pub fn setSecondaryEntry(entry: *const fn () callconv(.c) noreturn) void {
     smp.setSecondaryEntry(entry);
 }
 
+/// Test hook: force the next `n` AP wake attempts to fail, so the retry path can be
+/// exercised deterministically (see the smp-retry test). No effect when `n` is 0.
+pub fn testFailNextWakes(n: u32) void {
+    smp.testFailNextWakes(n);
+}
+
 /// Kernel tick rate: 1000 Hz (1 ms), the scheduler's time quantum.
 pub const timer_hz = 1000;