allocate AP kernel/IST stacks at bring-up, not statically

Hoist max_cpus into danos (root.zig), raise it to 128, and stop reserving a static [max_cpus][16 KiB] IST array. Only the BSP's IST stack is static (needed before the allocator); each AP's is heap-allocated when it comes online. Shrinks the kernel from ~1.1 MB to ~139 KiB in ReleaseSmall.

src/kernel/arch/x86_64/cpu.zig

@@ -124,6 +124,14 @@ pub fn setSecondaryEntry(entry: *const fn () callconv(.c) noreturn) void {
     smp.setSecondaryEntry(entry);
 }
 
+/// Bytes the kernel should allocate for an AP's IST (double-fault) stack, and where
+/// to record its top before waking the core. The stack is heap-allocated per online
+/// AP (the BSP's is static — it's needed before the allocator exists). See tss.zig.
+pub const ist_stack_size = tss.ist_stack_size;
+pub fn setApIstStack(cpu: usize, top: usize) void {
+    tss.setApIstStack(cpu, top);
+}
+
 /// 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 {