feat(v4.0.0): a node looks before it writes; two neighbours no longer stop each other

MESH.md step 4, the fault found there, as ruled (section 7a): a node
writes only to a neighbour that is reading, keeps what it takes in
meanwhile, and loses and counts what it has no room for.

Engine: two more addresses after a node's ports -- which ports have a
neighbour waiting to write to it, which to read from it. Nucleus: (GATE)
before every message; the messages waiting, a ring of 400 cells, dealt
with when the node is idle. Of two neighbours the lower number may wait
to write (ruled after it was built); NEIGHBOUR tells a node who is on
each port.

test_host_mesh.c, 44 checks: the case that stopped the nodes passes; six
messages from each node to each at once all arrive; 800 at once, the
nodes come to rest and every message arrived or was counted (287 arrived,
714 of all kinds let go). test_fabric.c: the two looks. Both widths,
ASan+UBSan.

POST: twelve cases handed HERE, a cell address on v4, to words that take
a byte address, and so wrote into or read from the nucleus's code at cell
HERE/4. Ruled: left out, marked OPEN, until HERE and the byte words are
made to agree as its own step. POST is 538 cases.

hosted-check on three ISAs, 538/538. Bare metal: logs/20261006-134817
(amd64), -135044 (aarch64), -135440 (riscv64).

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

v4/tools/mkimage.c

@@ -99,7 +99,7 @@ int main(int argc, char **argv)
     if (!v4_node_guards_intact(&n)) die("the node's guards are damaged", NULL);
 
     n.mem[LINE_STATUS] = 1;                          /* nothing has ended badly */
-    n.mem[OUT_PTR] = OUT_W;                          /* nothing has been printed */
+    HOST_MESSAGES_EMPTY(&n);                         /* nothing has been printed, no message waits */
 
     /* the nucleus as a capsule of F18 code: the words a neighbour writes to
      * an empty node's port, each in V4_CELL_BITS / 8 bytes, low byte first */