serial: skip a dead COM1 via a loopback probe (fixes ~57s real-HW boot)

On a legacy-free board COM1 is decoded but has no live UART: its LSR reads 0x00, so the transmit-holding-empty bit never sets and writeByte spun its full 100k-iteration guard on every logged byte (~15ms/byte x ~3.7k bytes to 'initialised' ~= 57s). QEMU always has a working UART, so this only bit real hardware; the boot log survived via log.ramSink.

- probe() loopback-tests the UART in init(); write() is a no-op when absent, so a dead port costs nothing per byte
- guard cut 100k -> 5k (backstop for a live-but-stalled UART only)
- serialPresent() + a boot line making the absent-UART case visible

system/kernel/kernel.zig

@@ -87,6 +87,10 @@ fn kmain(boot_information: *const BootInformation) noreturn {
         "/system/kernel: framebuffer console online (bootstrap; graphics driver later)\n"
     else
         "/system/kernel: no framebuffer (headless) -> logging to serial/debugcon only\n");
+    log.write(if (architecture.serialPresent())
+        "/system/kernel: serial console online (COM1)\n"
+    else
+        "/system/kernel: no serial UART (COM1 absent) -> log kept in RAM/debugcon\n");
     log.write("/system/kernel: cpu tables online (GDT, IDT, TSS)\n");
     log.print("  resolution : {d}x{d}\n", .{ fb.width, fb.height });
     log.print("  pitch      : {d} bytes\n", .{fb.pitch});