build: v4 is the default build -- every kernel boots v4; acceptance is v4 on three ISAs and the 32-bit build

Captain Bob, 2026-10-14: "v4 is the default build", "no, qemu to v4
too ... we are effectively trying to retire v3", "the 32 bit build needs
to go into the acceptance test too", "i don't care about v3".

- STARFORTH_V4 defaults on (Kconfig default y) and V4_DEFAULT=1
  everywhere: every kernel carries v4 and v3 and boots v4; system=v3 boots
  v3; STARFORTH_V4=0 builds v3 alone.  Board images write system=v4.
- Acceptance (CLAUDE.md): the three ISAs as built (v4, POST passed,
  V4_SYSTEM dict_hash equal) and make -C v4 test-32.  v3 is not part of
  it.

Acceptance run: amd64, aarch64, riscv64 -- PARITY:V4_POST 538/538,
dict_hash=0xe5a57f3eb72ff94a on all three, no unknown word; test-32: 30
suites, 0 failures.  The SER5 image under OVMF boots v4 as built, v3 after
the one-word edit.  README, BRINGUP, FABRIC-3 XLIII, the register updated.

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

README.md

@@ -21,7 +21,8 @@ StarForth v4 moves the engine onto a 32-instruction core derived from Chuck Moor
 FORTH-79 is recomposed on the F18 engine and stored as a capsule; the StarshipOS-specific
 vocabulary gets the same treatment. v3 remains the reference system until v4 meets its
 acceptance criteria (`docs/v4.0.0/JUSTIFICATION.md` §16). **Both run on bare metal now:** every
-board image carries v3 and v4, and one word in its boot configuration chooses (see below).
+board image carries v4 and v3, boots v4 as built, and one word in its boot configuration
+chooses (see below).
 
 ---
 
@@ -36,8 +37,10 @@ board image carries v3 and v4, and one word in its boot configuration chooses (s
 | M9 | Block storage — Artemis's disk on virtio-blk; identity thumbdrives on USB mass storage | ✅ In QEMU · real disks: no driver yet (`boards/BRINGUP.md` A4) |
 | Bare metal | SER5 (amd64), Raspberry Pi 5 (aarch64), Milk-V Mars (riscv64); Zynq-7020 (ARMv7) not yet ported | Images build; first boots on the bench — `boards/BRINGUP.md` |
 
-POST at boot, on all three ISAs: **1052 tests**, parity hash verified —
-`PARITY:M7.1a word_count=526 hash=0x82f4ae7cb2e2d424`. The v4 system's POST: 538 tests.
+**v4 is the default build, and v3 is being retired**: every kernel boots v4 unless its boot
+arguments say `system=v3`. POST at boot, on all three ISAs: v4 — **538 tests**,
+`PARITY:V4_SYSTEM word_count=317 dict_hash=0xe5a57f3eb72ff94a`; v3 — **1052 tests**,
+`PARITY:M7.1a word_count=526 hash=0x82f4ae7cb2e2d424`.
 
 ### What's live
 
@@ -72,8 +75,8 @@ make boot_image TARGET=RASPI      # Raspberry Pi 5 -- FAT, config.txt + kernel_2
 make boot_image TARGET=MILKV      # Milk-V Mars    -- GPT/ESP, BOOTRISCV64.EFI (U-Boot bootefi)
 make boards                       # list boards (the Zynq-7020 has no ARMv7 port yet)
 
-# Each image carries v3 and v4. As built it boots v3; to boot v4, change the one word
-#   system=v3  ->  system=v4
+# Each image carries v4 and v3. As built it boots v4; to boot v3, change the one word
+#   system=v4  ->  system=v3
 # in starforth.cfg on the SER5's stick or the Mars's card, or cmdline.txt on the Pi 5's card.
 
 # The same TARGET drives all, docs and clean (make help)
@@ -81,11 +84,11 @@ make                              # every board with a kernel port + hosted v3 +
 make docs                         # build/docs/LithosAnanke.pdf (LaTeX master: docs/book/main.tex)
 make clean TARGET=RASPI           # that board's outputs and build/aarch64/kernel
 
-# The kernel in QEMU (OVMF / AAVMF / EDK2), one architecture at a time
+# The kernel in QEMU (OVMF / AAVMF / EDK2), one architecture at a time -- boots v4
 make -f kernel/Makefile ARCH=amd64   clean qemu
 make -f kernel/Makefile ARCH=aarch64 clean qemu
 make -f kernel/Makefile ARCH=riscv64 clean qemu
-make -f kernel/Makefile ARCH=amd64 STARFORTH_V4=1 clean qemu    # the v4 system
+make -f kernel/Makefile ARCH=amd64 KERNEL_ARGS=system=v3 clean qemu   # v3, while it remains
 
 # The Pi 5's native boot path, rehearsed on QEMU's virt machine at EL2
 make -f kernel/Makefile ARCH=aarch64 RPI5_NATIVE_BASE=0x40080000 rpi5-rehearsal