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/cpu.zig

@@ -5,6 +5,7 @@
 //! (halt, the descriptor tables, later paging), and nothing generic.
 
 const danos = @import("danos");
+const config = @import("config");
 const gdt = @import("gdt.zig");
 const tss = @import("tss.zig");
 const idt = @import("idt.zig");
@@ -148,8 +149,8 @@ pub fn pageExecutable(virt: u64) bool {
     return paging.isExecutable(virt);
 }
 
-/// Kernel tick rate: 1000 Hz (1 ms), the scheduler's time quantum.
-pub const timer_hz = 1000;
+/// Kernel tick rate (the scheduler's time quantum), from config.
+pub const timer_hz = config.timer_hz;
 
 /// The ACPI PM timer, as a calibration reference (re-exported for the config).
 pub const PmTimer = apic.PmTimer;