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/scheduler.zig

@@ -18,6 +18,7 @@
 //! shared queues.
 
 const std = @import("std");
+const danos = @import("danos");
 const arch = @import("arch");
 const heap = @import("heap.zig");
 const sync = @import("sync.zig");
@@ -68,7 +69,7 @@ pub const PerCpu = struct {
     pinned_bitmap: u8 = 0,
 };
 
-const max_cpus = 64; // matches the discovery pool (src/device/acpi.zig)
+const max_cpus = danos.max_cpus;
 var cpus = [_]PerCpu{.{}} ** max_cpus;
 
 /// This core's per-CPU state, via the arch layer's GS-base pointer. Valid only once