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/include/v4/node.h

@@ -109,6 +109,8 @@ typedef struct {
     v4_cell       given_value;
     unsigned      given_port;
     unsigned      last_from;         /* the port the last read from "any" took its word from */
+    unsigned      writers;           /* bit k: the neighbour on port k is blocked writing to this node */
+    unsigned      readers;           /* bit k: the neighbour on port k is blocked reading from this node */
 
     /* DSTACK-DEPTH and RSTACK-DEPTH (D-16).  See v4_node_stack_regs_attach. */
     v4_cell       dstack_reg;        /* its word address, or -1 */
@@ -181,6 +183,13 @@ void v4_node_console_attach(v4_node *n, v4_cell addr);
  *   base + V4_PORTS       "any port": a read here takes from whichever port
  *                         has a neighbour writing
  *   base + V4_PORTS + 1   which port the last read from "any" came from
+ *   base + V4_PORTS + 2   which ports have a neighbour waiting to write to this
+ *                         node: bit k for port k
+ *   base + V4_PORTS + 3   which ports have a neighbour waiting to read from it
+ * The last two are how a node looks before it writes (MESH.md 7a), as the
+ * F18's io register lets it.  They are what whoever connects the ports last
+ * said, v4_node_port_status; fetching them waits for nothing and changes
+ * nothing.
  *                         (read only)
  * None of them is memory while the ports are attached.
  *
@@ -213,6 +222,11 @@ void v4_node_port_served(v4_node *n);
  * port or "any", is unblocked and will fetch it. */
 void v4_node_port_give(v4_node *n, unsigned port, v4_cell value);
 
+/* What is on the other end of the node's ports now: bit k of `writers` if
+ * the neighbour on port k is blocked writing to this node, bit k of
+ * `readers` if it is blocked reading from it and would take a word. */
+void v4_node_port_status(v4_node *n, unsigned writers, unsigned readers);
+
 /* Which port `addr` is: 0 .. V4_PORTS - 1, V4_PORT_ANY, V4_PORTS + 1 for the
  * "which port" address, or -1 if it is none of them. */
 int v4_node_port_index(const v4_node *n, v4_cell addr);