serial: make the log sink a build option, off by default

Serial is now a QEMU/dev aid, not a real-hardware necessity: a legacy-free board often has no live COM1, and the boot log is kept in RAM (klog) and flushed to disk. So the serial sink is compiled in only under -Dserial (default false).

- kernel.zig gates serialInit + the log sink on build_options.serial
- boot/efi.zig gates its EFI: progress breadcrumbs (con_out) via progress(); fatal-error messages stay always-on so a failed boot still explains itself
- run-x86-64 boots a serial-enabled image variant (factored addKernel/addBootImage helpers) so a dev boot always captures serial0, without baking serial into the flashable image
- test/qemu_test.py builds -Dserial=true (it asserts on serial markers)
- the loopback probe stays as a real-HW safety net for -Dserial images

test/qemu_test.py

@@ -504,7 +504,10 @@ TIMEOUT = 30  # seconds per case
 
 
 def build(arch, case):
-    cmd = ["zig", "build", f"-Dtest-case={case}"] + arch["zig_flags"]
+    # -Dserial: the harness asserts on markers the kernel writes to serial0, so the
+    # serial log sink must be compiled in. It is off by default (a flashed real-
+    # hardware image keeps its log in RAM instead; see build.zig / serial.zig).
+    cmd = ["zig", "build", f"-Dtest-case={case}", "-Dserial=true"] + arch["zig_flags"]
     r = subprocess.run(cmd, cwd=REPO, capture_output=True, text=True)
     if r.returncode != 0:
         return r.stderr.strip() or r.stdout.strip()