feat(boards): both systems on every board image, chosen at boot -- system=v3 or system=v4

boards/BRINGUP.md A6, FABRIC-3 SXLIII (ruled: one image, switch at boot).
A STARFORTH_V4 kernel carries v3 too; system= in the boot arguments
chooses, else the build's default (V4_DEFAULT: 1 for plain STARFORTH_V4
builds as before, 0 for board images).  make boot_image builds both and
always writes starforth.cfg / cmdline.txt with system=v3 first, and cleans
its ISA's objects first -- that clean was removing build/<arch>/SER5, not
the kernel's objects, until fixed here.

Each ISA: v3 by default (hash unchanged), v4 with system=v4 (538/538).
The SER5 image under OVMF: v3 as built, v4 after a one-word edit on it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

kernel/include/starkernel/cmdline.h

@@ -23,6 +23,8 @@
  *   --log-level=<level>      debug / info / warn / error
  *   --stack=<N>[KMG]         stack_size in bytes
  *   --heap=<N>[KMG]          heap_size in bytes
+ *   system=v3 | system=v4    which system runs, where both are built in
+ *                            ("--system=" too)
  */
 void cmdline_parse_ascii(const char *cmdline, KernelArgs *args);