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/src/image.c

@@ -1,34 +1,26 @@
-/* image.c -- start a node from a capsule image.  See image.h.
+/* image.c -- the StarForth nucleus, as its host knows it.  See image.h.
  *
  * Nothing here uses the C library: the bare-metal kernel links this file.
  */
 #include "v4/image.h"
 
-int v4_image_boot(v4_node *n, v4_exec_state *es, v4_heat *h, const v4_image *im,
+int v4_image_born(v4_node *n, v4_exec_state *es, v4_heat *h, const v4_image *im,
                   unsigned char *disk, unsigned blocks)
 {
-    unsigned i;
-
     if (im->cell_bits != (unsigned)V4_CELL_BITS || im->node_words != (unsigned)V4_NODE_WORDS
         || im->data_ring != (unsigned)V4_DATA_RING || im->ret_ring != (unsigned)V4_RET_RING)
         return 0;
 
-    v4_node_reset(n);
+    v4_node_born(n, im->port);
     v4_exec_reset(es);
     v4_heat_reset(h);
-    for (i = 0; i < im->count; i++)
-        if (v4_node_addr_ok(im->cells[i].addr)) n->mem[im->cells[i].addr] = im->cells[i].value;
 
     v4_node_console_attach(n, im->console_tx);
     v4_node_console_input_attach(n, im->console_rx, im->console_status);
     v4_node_stack_regs_attach(n, im->dstack_reg, im->rstack_reg);
     v4_node_error_attach(n, im->node_error);
     v4_node_fault_attach(n, im->fault_table);
-    v4_node_port_attach(n, im->port);
     if (disk && blocks) v4_node_storage_attach(n, im->storage_reg, disk, blocks);
-
-    n->mem[im->line_status] = V4_LINE_COMPLETED;
-    n->p = im->idle;
     return 1;
 }