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/main.zig

@@ -1,5 +1,6 @@
 const std = @import("std");
 const danos = @import("danos");
+const config = @import("config");
 const arch = @import("arch");
 const console = @import("console.zig");
 const log = @import("log.zig");
@@ -278,7 +279,7 @@ fn bringUpSecondaries() void {
     log.print("\ndanos: bringing up {d} application processor(s)\n", .{cores.len - 1});
     const max_wake_attempts = 3; // a core that misses the first INIT-SIPI-SIPI gets retried
     for (cores[1..], 1..) |core, index| {
-        const stack = heap.allocator().alloc(u8, 16 * 1024) catch {
+        const stack = heap.allocator().alloc(u8, config.kernel_stack_size) catch {
             log.print("  cpu apic_id {d}: no stack; skipped\n", .{core.apic_id});
             continue;
         };