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;
 

src/kernel/arch/x86_64/smp.zig

@@ -51,6 +51,14 @@ pub fn setSecondaryEntry(entry: *const fn () callconv(.c) noreturn) void {
     secondary_entry = entry;
 }
 
+/// Test hook: force the next `n` wake attempts to fail (skipping the actual
+/// INIT-SIPI-SIPI), so the retry path can be exercised deterministically. Zero in
+/// normal operation — the smp-retry test arms it via `arch.testFailNextWakes`.
+var fail_next_wakes: u32 = 0;
+pub fn testFailNextWakes(n: u32) void {
+    fail_next_wakes = n;
+}
+
 /// Record the reserved low frame the trampoline uses. Call once at boot. The frame
 /// starts inert (identity-mapped RW+NX like all RAM); each wake arms it and disarms
 /// it again, so it's only ever executable while a core is climbing.
@@ -101,6 +109,11 @@ pub fn startAp(apic_id: u32, stack_top: usize, percpu: usize, index: usize, cr3:
     arm();
     defer disarm();
 
+    if (fail_next_wakes > 0) { // test hook: simulate a core missing this attempt
+        fail_next_wakes -= 1;
+        return false;
+    }
+
     boot_index = index;
     param("ap_tramp_cr3").* = cr3;
     param("ap_tramp_stack").* = stack_top;