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.

build.zig

@@ -468,6 +468,10 @@ pub fn build(b: *std.Build) void {
     // QEMU test harness (test/qemu_test.py, which asserts on serial markers) turn
     // it on; a flashable `zig build` image leaves it out. See serial.zig.
     const serial = b.option(bool, "serial", "Compile the serial-console log sink into the kernel (default: off; run-x86-64 and the test harness enable it)") orelse false;
+    // The diagnose boot: init skips the display service (and demo), so the
+    // on-screen boot transcript is never suppressed — the full timestamped
+    // timeline stays on the screen for real-hardware debugging by eye.
+    const diagnose = b.option(bool, "diagnose", "Boot without the display service so the timestamped boot transcript stays on screen (real-hardware debugging)") orelse false;
 
     // --- Kernel: freestanding x86_64 ELF, jumped to by the bootloader ---
     // SSE2 is part of the x86_64 baseline and UEFI leaves it enabled at handoff,
@@ -511,6 +515,7 @@ pub fn build(b: *std.Build) void {
     // the heartbeat stays present under test.
     const init_options = b.addOptions();
     init_options.addOption(bool, "serial", serial);
+    init_options.addOption(bool, "diagnose", diagnose);
     programModule(init_exe).addImport("build_options", init_options.createModule());
 
     // --- the rest of the /system tree: services, drivers, test fixtures ---