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/main.zig

@@ -283,6 +283,13 @@ fn bringUpSecondaries() void {
             continue;
         };
         const stack_top = (@intFromPtr(stack.ptr) + stack.len) & ~@as(usize, 15);
+        // This core's IST (double-fault) stack — allocated only now that the core is
+        // real, rather than reserved statically for every possible core.
+        const ist = heap.allocator().alloc(u8, arch.ist_stack_size) catch {
+            log.print("  cpu apic_id {d}: no IST stack; skipped\n", .{core.apic_id});
+            continue;
+        };
+        arch.setApIstStack(index, (@intFromPtr(ist.ptr) + ist.len) & ~@as(usize, 15));
         const pc = scheduler.prepareSecondary(index, core.apic_id);
         var attempt: u32 = 1;
         while (attempt <= max_wake_attempts) : (attempt += 1) {