gather kernel tunables into config.zig

max_cpus, max_tasks, kernel/IST stack sizes, and timer_hz move from scattered constants into one config module. root.zig goes back to being just the boot contract. Values unchanged; any can become a -D build option later.

src/root.zig

@@ -45,14 +45,6 @@ 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