fix(v4.0.0): step 6c mended after its review -- what a stuck or gone node can and cannot hold up

The review of step 6c found it did not keep its rule.  Ruled: fix them
all; 16 hops.

- Paying a NACK no longer waits on a lower-numbered neighbour; a node
  that owes one does not sleep, and goes on taking in what it is sent.
- AWAIT looks first through the messages already waiting.
- A node waiting to write to a removed neighbour gets error 18: the
  word that says who is reading also says which ports have anything on
  them.
- A console line lets Hera go when she is blocked on the first word of
  a message (v4_fabric_interrupt_error, a device's `pending`).
- A refused answer is told to the node that waits, not the one that
  answered; one owed to the node itself goes to its own queue.
- A message is passed on by at most 16 nodes, then refused.
- A GONE is believed only when it says it is from the node's centre.
- Tests: the check that tested nothing is replaced; no room for a
  waiting sender; the limit that remains is tested as a limit.
- MESH.md 7b's rule reworded to what holds, 7b.7 says where it falls
  short; README likewise.

make -C v4 test, sanitize, hosted-check; three bare-metal boots.  POST
538 of 538, word_count=317, dict_hash=0xc0769523a47b7dc3 on all six.

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

v4/src/fabric.c

@@ -18,6 +18,7 @@ void v4_fabric_init(v4_fabric *f, v4_place *places, unsigned capacity)
     f->place = places;
     f->capacity = capacity;
     f->gone_error = 0;
+    f->interrupt_error = 0;
     for (i = 0; i < capacity; i++) clear_place(&places[i]);
 }
 
@@ -26,6 +27,11 @@ void v4_fabric_gone_error(v4_fabric *f, v4_cell code)
     f->gone_error = code;
 }
 
+void v4_fabric_interrupt_error(v4_fabric *f, v4_cell code)
+{
+    f->interrupt_error = code;
+}
+
 void v4_fabric_grow(v4_fabric *f, v4_place *places, unsigned capacity)
 {
     unsigned i;
@@ -171,6 +177,7 @@ static void look_about(v4_fabric *f, unsigned a)
 
     for (k = 0; k < V4_PORTS; k++) {
         const v4_wire *w = &f->place[a].wire[k];
+        if (w->kind != V4_WIRE_NONE) readers |= 1u << (V4_PORTS + k);    /* there is something on this port */
         if (w->kind == V4_WIRE_DEVICE) {
             if (w->device->take) readers |= 1u << k;
         } else if (w->kind == V4_WIRE_NODE && awake(f, w->node)) {
@@ -202,6 +209,24 @@ unsigned v4_fabric_step(v4_fabric *f)
     for (i = 0; i < f->capacity; i++)
         if (awake(f, i) && f->place[i].node->n.reading && !f->place[i].node->n.given) done += hand_over_read(f, i);
 
+    /* a node blocked on the first word of a message to a node that is not
+     * reading, while a device of its own has something for it */
+    if (f->interrupt_error != 0)
+        for (i = 0; i < f->capacity; i++) {
+            v4_node *n;
+            unsigned k;
+            if (!awake(f, i)) continue;
+            n = &f->place[i].node->n;
+            if (!n->asking || f->place[i].wire[n->ask_port].kind != V4_WIRE_NODE || v4_node_words_since_look(n) != 0) continue;
+            for (k = 0; k < V4_PORTS; k++) {
+                const v4_wire *w = &f->place[i].wire[k];
+                if (w->kind == V4_WIRE_DEVICE && w->device->pending && w->device->pending(w->device->self)) {
+                    done += (unsigned)v4_node_port_gone(n, f->interrupt_error);
+                    break;
+                }
+            }
+        }
+
     /* a node blocked at one port that nothing is wired to */
     if (f->gone_error != 0)
         for (i = 0; i < f->capacity; i++) {