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/README.md

@@ -133,13 +133,16 @@ The two products are not yet this: they become the five nodes at steps 8
 and 9.
 A node she births is not POSTed: POST is the kernel's, once, on Hera.
 
-**Refusals and waits** (`MESH.md` 7b, step 6c). No message is lost without
-its sender being told, and no node waits for ever. A node with no room for
-a message, or no way to pass it on, sends its sender a NACK; `(REFUSED)`
+**Refusals and waits** (`MESH.md` 7b, step 6c). A node is never left
+waiting on a node that is gone, and a wait on a node that is alive and
+stuck is ended by Hera's killing it, or from the console. A node with no
+room for a message, or no way to pass it on, or that would be the
+sixteenth to pass it on, sends a NACK to whoever waits on it; `(REFUSED)`
 counts them. `AWAIT` ends in an error if the answer is not going to come:
 "Message refused", "Node gone", or "Interrupted" when a line is typed at
 the console. `n KILL` on Hera removes node n and tells every other node it
-is gone.
+is gone; a node believes that only from its centre. A stuck node can hold
+up its neighbours until it is killed: `MESH.md` 7b.7 says how.
 
 **Blocks** (`MESH.md` step 6). The block words are the kernel's, as they
 are for a v3 VM: `BLOCK`, `BUFFER`, `UPDATE`, `SAVE-BUFFERS` and