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/node.c

@@ -120,8 +120,8 @@ v4_cell v4_node_fetch(v4_node *n, v4_cell addr)
     {
         int port = v4_node_port_index(n, addr);
         if (port == (int)V4_PORTS + 1) return (v4_cell)n->last_from;
-        if (port == (int)V4_PORTS + 2) return (v4_cell)n->writers;
-        if (port == (int)V4_PORTS + 3) return (v4_cell)n->readers;
+        if (port == (int)V4_PORTS + 2) { n->since_look = 0; return (v4_cell)n->writers; }
+        if (port == (int)V4_PORTS + 3) { n->since_look = 0; return (v4_cell)n->readers; }
         if (port >= 0) {
             /* the word its neighbour wrote; v4_node_read_ready has said it is there */
             if (!n->given) return 0;
@@ -179,6 +179,7 @@ void v4_node_port_attach(v4_node *n, v4_cell base)
     n->last_from = 0;
     n->writers = 0;
     n->readers = 0;
+    n->since_look = 0;
 }
 
 void v4_node_port_status(v4_node *n, unsigned writers, unsigned readers)
@@ -221,6 +222,12 @@ void v4_node_born(v4_node *n, v4_cell base)
 void v4_node_port_served(v4_node *n)
 {
     n->asking = 0;
+    n->since_look++;
+}
+
+unsigned v4_node_words_since_look(const v4_node *n)
+{
+    return n->since_look;
 }
 
 int v4_node_port_gone(v4_node *n, v4_cell code)