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/system/boot.c

@@ -354,7 +354,7 @@ int v4_boot_run(const v4_boot *b)
     /* what is on its two ports, the kernel and the console, takes a word
      * when it is written: the node need never wait before writing to them
      * (node.h, v4_node_port_status) */
-    v4_node_port_status(b->n, 0, 1u << KERNEL_PORT | 1u << CONSOLE_PORT);
+    v4_node_port_status(b->n, 0, (1u << KERNEL_PORT | 1u << CONSOLE_PORT) * (1u + (1u << V4_PORTS)));   /* each of them there, and taking */
     v4_blocks_init(&boot_blocks, b->im->block_window);
     if (!load_nucleus(b)) { say(b, "PARITY:FAIL\nPOST: FAILED\n"); return 0; }