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 */

v4/tools/post79_rules.py

@@ -67,6 +67,19 @@ STANDARD = {
     "MOVE.basic": dict(text="65 HERE ! 66 HERE 1+ ! HERE HERE 16 + 2 MOVE HERE 16 + @ . HERE 17 + @ . CR",
                        out="65 66 \n", why="FORTH-79: MOVE moves cells; and " + D1),
     "PAD.usable": None,     # see OPEN below
+    # see OPEN below: HERE handed to a word that takes a byte address
+    "C!.basic": None,
+    "C!.zero": None,
+    "C!.high_byte": None,
+    "C!.truncation": None,
+    "C@.after_cstore": None,
+    "C@.zero_byte": None,
+    "FILL.basic": None,
+    "FILL.zero_byte": None,
+    "CMOVE.overlap": None,
+    "TYPE.single_char": None,
+    "COUNT.string_bounds": None,
+    "COUNT.zero_addr_plus_one": None,
 
     "BASE.base_store": dict(out="255 \n", why="255 read in base 16 and printed in base 16 is 255; v3 printed 467"),
 
@@ -123,9 +136,29 @@ STANDARD = {
     "LEAVE.qdloop": dict(out="3 \n", why=LEAVE),
 }
 
+# Found 2026-10-06 (docs/v4.0.0/MESH.md section 10, step 4): on v4 HERE is a
+# cell address and C! C@ FILL CMOVE COUNT TYPE take byte addresses, so these
+# cases wrote into, or read from, the nucleus's own code at cell HERE/4 --
+# and passed, reading back what they had written there.  Ruled: left out
+# until how HERE and the byte words agree is settled, as its own step.
+HERE_BYTE = ("OPEN: HERE is a cell address on v4 and this word takes a byte address (D-1): "
+             "the case wrote to or read from the nucleus's code at cell HERE/4")
+
 # Dropped, and why.  mkpost.py prints these with the cases it leaves out.
 DROPPED = {
     "PAD.usable": "OPEN: PAD 42 OVER ! faults on v4 -- PAD is a byte address and ! takes a cell address (D-1)",
+    "C!.basic": HERE_BYTE,
+    "C!.zero": HERE_BYTE,
+    "C!.high_byte": HERE_BYTE,
+    "C!.truncation": HERE_BYTE,
+    "C@.after_cstore": HERE_BYTE,
+    "C@.zero_byte": HERE_BYTE,
+    "FILL.basic": HERE_BYTE,
+    "FILL.zero_byte": HERE_BYTE,
+    "CMOVE.overlap": HERE_BYTE,
+    "TYPE.single_char": HERE_BYTE,
+    "COUNT.string_bounds": HERE_BYTE,
+    "COUNT.zero_addr_plus_one": HERE_BYTE,
     "CMOVE.bounds": "moves 1000 bytes over the system; v3 refuses by a bounds rule FORTH-79 has not got",
 }
 for _name in [k for k, v in STANDARD.items() if v is None and k.startswith("WORD.")]: