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/kernel/arch/x86_64/tss.zig

@@ -9,7 +9,7 @@
 //! once can't share one fault stack. So the TSS and its IST stack are per-core,
 //! indexed by CPU number; slot 0 is the BSP.
 
-const danos = @import("danos");
+const config = @import("config");
 const gdt = @import("gdt.zig");
 
 /// x86_64 TSS. `packed` because several 64-bit fields sit at 4-byte-unaligned
@@ -35,8 +35,8 @@ const Tss = packed struct {
 /// The IST slot (1-based, as the IDT gate encodes it) used for critical faults.
 pub const double_fault_ist = 1;
 
-const max_cpus = danos.max_cpus;
-pub const ist_stack_size = 16 * 1024;
+const max_cpus = config.max_cpus;
+pub const ist_stack_size = config.ist_stack_size;
 
 /// One TSS per core (small — kept static). The IST stacks are 16 KiB each, so only
 /// the **BSP's** is static: it must exist before the frame allocator does, to catch a