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/README.md

@@ -61,7 +61,7 @@ load `v4:forth79.4th`, pass POST and reach `ok>`, with the same lines:
 ```
 PARITY:V4_NUCLEUS name=v4:nucleus-64.f18 capsule_id=... capsule_hash=0x97a655081b2af559 words=298
 PARITY:V4_CAPSULE name=v4:forth79.4th capsule_id=... dict_hash=...
-PARITY:V4_POST tests=550 pass=550 fail=0
+PARITY:V4_POST tests=538 pass=538 fail=0
 PARITY:V4_CAPSULE name=v4:post79.4th capsule_id=... dict_hash=...
 PARITY:OK
 POST: PASSED
@@ -115,11 +115,13 @@ steps 8 and 9.
 **Finding the way** (`MESH.md` step 4) is built: a node has a table of
 which port leads toward which node (`ROUTE`, `DEFAULT-ROUTE`, `NO-ROUTES`),
 passes on a message that is not for it, and `SEND`s text to another node;
-`v4/tests/test_host_mesh.c` runs three StarForth nodes in a row. **It has
-a fault that stands:** two neighbours that write to each other at the same
-moment wait for ever, because a write blocks until the neighbour reads and
-a node that is writing is not reading. The test shows it, named KNOWN
-FAULT. The ways out are in `MESH.md` section 7a and none is chosen yet.
+`v4/tests/test_host_mesh.c` runs three StarForth nodes in a row. A node
+looks before it begins a message (`MESH.md` section 7a): it takes in what
+its neighbours are waiting to write to it, keeps those messages until it
+has nothing else to do, and writes only to a neighbour that is waiting to
+read -- or, of two neighbours, the one with the lower number may wait.
+`NEIGHBOUR ( node port -- )` tells a node who is on each port. A node with
+no room for one more message lets it go and counts it in `(LOST)`.
 
 **This is still the lone node.** On bare metal `kernel_main.c` starts it
 before the fleet tables, beside the kernel's own system and not in the VM's
@@ -133,7 +135,7 @@ a time with POST after each, is the next step (`NUCLEUS.md` section 8).
 
 ## POST
 
-`capsules/v4/post79.4th`: 550 cases in blocks 7000 up, covering 126 of the
+`capsules/v4/post79.4th`: 538 cases in blocks 7000 up, covering 126 of the
 130 words of the FORTH-79 Required Word Set. 443 are v3's cases with what
 the hosted v3 binary did as the expected result. The other 107, and the 27
 v3 cases left out, are listed with reasons in `docs/v4.0.0/POST79.md`.