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/tests/host_map.h

@@ -64,6 +64,10 @@
 #define BLK          (BVARS + 7)
 #define SRC          (BVARS + 8)
 #define SRC_HOOK     (BVARS + 9)
+#define QUIET        (BUF0_W - 10)              /* 1 while the node does something that is no text's doing: an error then is not told (quit.v4, (RAISED)) */
+#define A_WORD       (BUF0_W - 113)             /* AWAIT's look through the messages waiting: the answer's word, */
+#define A_LEFT       (BUF0_W - 114)             /*   how many cells are still to be looked at, */
+#define A_FOUND      (BUF0_W - 115)             /*   and what was found */
 #define REFUSED      (BUF0_W - 7)               /* how many of this node's messages it has been told were refused */
 #define OWED_COUNT   (BUF0_W - 8)               /* how many refusals this node owes and has not sent yet, at most 8 */
 #define OWED         (BUF0_W - 112)             /* those: for each, the node it is owed to and the node the refused message was for */
@@ -196,6 +200,10 @@ static int host_load(v4_text *tx, v4_node *n, const char *const *files, unsigned
     v4_text_constant(tx, "(MQ)", MQ_W);
     v4_text_constant(tx, "(MQ-END)", MQ_W + MQ_CELLS);
     v4_text_constant(tx, "-MQ-ORDINARY", -(v4_cell)(MQ_CELLS - 7 - 36));   /* the same for a message that is not a refusal or a GONE: 36 cells are kept for those */
+    v4_text_constant(tx, "(QUIET)", QUIET);
+    v4_text_constant(tx, "(A-WORD)", A_WORD);
+    v4_text_constant(tx, "(A-LEFT)", A_LEFT);
+    v4_text_constant(tx, "(A-FOUND)", A_FOUND);
     v4_text_constant(tx, "(REFUSED)", REFUSED);
     v4_text_constant(tx, "(OWED#)", OWED_COUNT);
     v4_text_constant(tx, "(OWED)", OWED);