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/root.zig

@@ -45,6 +45,14 @@ pub const Framebuffer = extern struct {
 /// targets so far.
 pub const page_size = 4096;
 
+/// Ceiling on logical CPUs the kernel tracks — the size of the per-CPU bookkeeping
+/// arrays (discovery pool, scheduler state, per-core GDT/TSS). A generous headroom:
+/// these structs are small (a few hundred bytes each), and the *large* per-core
+/// resources (IST/kernel stacks) are allocated at bring-up, only for cores that
+/// actually come online — so this ceiling costs little. A machine with more logical
+/// CPUs than this has its surplus reported and left parked (see acpi `cpusDropped`).
+pub const max_cpus = 128;
+
 /// danos's own classification of a span of physical memory — deliberately not
 /// UEFI's vocabulary. Each boot path (UEFI now, device tree later) translates its
 /// native memory description into these kinds, so the kernel never learns what