build: -Ddiagnose — boot without the display so the transcript stays on screen

The display service claiming the framebuffer suppresses the on-screen
boot transcript — correctly in normal operation, but on a serial-less
machine being debugged, the timeline vanishes just when it matters. A
diagnose image (zig build -Ddiagnose=true) has init skip the display
service and demo: the timestamped transcript stays on screen
indefinitely, and the power button still runs the orderly shutdown (so
the logger's files land when storage works).

First use immediately caught a real bug IN QEMU: an intermittent (~1
in 3) usb-storage READ CAPACITY failure after a ~21 s stall — after
which usb-storage and fat both exit cleanly and nothing retries: one
transient early-boot USB failure leaves the system permanently without
storage (and therefore without logs). That no-retry policy is a prime
suspect for real hardware never mounting /var, and is Track B's first
work item.

system/services/init/init.zig

@@ -27,7 +27,15 @@ const build_options = @import("build_options");
 /// kernel. Drivers are absent on purpose: the device manager owns those. (A
 /// future init reads this from a manifest under /system/services instead of a
 /// hardcoded list.)
-const boot_services = [_][]const u8{
+const boot_services = if (build_options.diagnose) [_][]const u8{
+    // The diagnose boot: no display service, so the kernel's on-screen boot
+    // transcript is never suppressed — the timestamped timeline (USB bring-up,
+    // storage, logger) stays readable on real hardware with no serial.
+    "/system/services/input",
+    "/system/services/device-manager",
+    "/system/services/fat",
+    "/system/services/logger",
+} else [_][]const u8{
     "/system/services/input",
     "/system/services/device-manager",
     "/system/services/fat",