feat(v4.0.0): the nucleus is a capsule, sent to a node born empty

MESH.md step 2.  A capsule of F18 code is the words a neighbour writes to a
node's port: for each stretch of memory, "@p a! @p push", the address and
count, "@p !+ unext" and the words; then a jump to the start.  A node born
empty executes that from its port, so it needs nothing in it beforehand.

- capsule.h/.c: v4_capsule_write, any node's memory as such a capsule
- mkimage writes the nucleus so, to capsules/v4/nucleus-64.f18, and the
  addresses a host needs as a C file; the memory image is no longer linked
  into either product
- mkcapsule is unchanged: the nucleus capsule is a built file kept under
  capsules/, as BLOCK_MAP.md is, and is baked, hashed and signed with the
  rest
- boot: the node is born empty (v4_image_born); the nucleus capsule is
  found, its hash and signature checked, and given to the node a word at a
  time as it reads its port; PARITY:V4_NUCLEUS carries its name and hash

Verified: test_fabric.c (59 checks, both widths, and under ASan and UBSan):
a memory with a programme and scattered words arrives word for word in an
empty node and runs.  The nucleus capsule rebuilds byte for byte.
hosted-check passes on three ISAs; clean qemu with STARFORTH_V4=1 on amd64,
aarch64 and riscv64 takes the nucleus in, passes POST (550 of 550) and
answers lines typed at each prompt (logs/20261006-102421, -102706,
-103048).

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

v4/Makefile

@@ -60,8 +60,16 @@ capsule_dir := -DV4_CAPSULE_DIR='"$(HERE)/capsule"'
 # capsule/*.v4 -- and writes it as a C file, $(BINDIR)/v4_image_<width>.c.
 # The hosted system below and the bare-metal kernel (kernel/Makefile,
 # STARFORTH_V4=1) link the 64-bit one.  docs/v4.0.0/NUCLEUS.md.
+# The nucleus as a capsule of F18 code (include/v4/capsule.h).  The 64-bit one
+# is a product's: it goes into the capsule directory, ../capsules/v4, where
+# the repository's mkcapsule finds it, hashes it and signs it with every
+# other capsule.  It is a built file kept in the tree, as
+# ../capsules/BLOCK_MAP.md is, and is to be committed when it changes.
+ROOT        := $(abspath $(HERE)/..)
+NUCLEUS_64  := $(ROOT)/capsules/v4/nucleus-64.f18
+NUCLEUS_32  := $(BINDIR)/nucleus-32.f18
 HOST_DEFS   := -DV4_NODE_WORDS=$(HOST_WORDS) -DV4_DATA_RING=$(HOST_DATA_RING) -DV4_RET_RING=$(HOST_RET_RING)
-ENGINE_SRCS := $(addprefix $(HERE)/src/,node.c exec.c stack.c iword.c heat.c guard.c image.c fabric.c)
+ENGINE_SRCS := $(addprefix $(HERE)/src/,node.c exec.c stack.c iword.c heat.c guard.c image.c fabric.c capsule.c)
 
 define IMAGE_RULE
 $(BINDIR)/mkimage-$(1): $(HERE)/tools/mkimage.c $$(SRCS) $$(wildcard $(HERE)/include/v4/*.h) $(HERE)/tests/host_map.h $(HERE)/Makefile
@@ -69,7 +77,7 @@ $(BINDIR)/mkimage-$(1): $(HERE)/tools/mkimage.c $$(SRCS) $$(wildcard $(HERE)/inc
 	$$(CC) $$(CFLAGS) -I$(HERE)/include -DV4_CELL_BITS=$(1) $(HOST_DEFS) $(capsule_dir) $(HERE)/tools/mkimage.c $$(SRCS) -o $$@
 
 $(BINDIR)/v4_image_$(1).c: $(BINDIR)/mkimage-$(1) $$(CAPSULES)
-	$(BINDIR)/mkimage-$(1) $$@
+	$(BINDIR)/mkimage-$(1) $$@ $$(NUCLEUS_$(1))
 endef
 $(foreach w,$(WIDTHS),$(eval $(call IMAGE_RULE,$(w))))
 
@@ -85,7 +93,6 @@ image:  $(foreach w,$(WIDTHS),$(BINDIR)/v4_image_$(w).c)
 # SIGN_KEY), and the code that reads it is the kernel's, compiled here as it
 # is there.  The binaries are static, so the two foreign ones run under
 # user-mode QEMU with nothing else installed.
-ROOT          := $(abspath $(HERE)/..)
 HOSTED_ISAS   := amd64 aarch64 riscv64
 CC_amd64      ?= cc
 CC_aarch64    ?= aarch64-linux-gnu-gcc
@@ -114,7 +121,7 @@ $(BINDIR)/mkcapsule: $(MKCAPSULE_SRCS)
 	@mkdir -p $(BINDIR)
 	cc -std=c99 -Wall -Wextra -O2 -I$(ROOT)/v3/include -I$(ROOT)/kernel/include -I$(ROOT)/tools -o $@ $(MKCAPSULE_SRCS)
 
-$(CAPSULE_DIR_C): $(BINDIR)/mkcapsule $(CAPSULE_FILES)
+$(CAPSULE_DIR_C): $(BINDIR)/mkcapsule $(CAPSULE_FILES) $(BINDIR)/v4_image_64.c
 	$(BINDIR)/mkcapsule $(SIGN_KEY_ARGS) $(ROOT)/capsules $@
 
 define HOSTED_RULE