feat(v4.0.0): a refused message is told to its sender -- NACK and GONE

A node that has no room for a message, or no way to pass it on, lets it
go and counts it as before, and now notes that it owes its sender a NACK
(type 4: the node the message was for).  It sends what it owes when it
next has nothing else to do.  36 cells of the messages waiting are kept
for NACK and GONE, and nothing is owed for either.

A node sent a NACK counts it in (REFUSED); one waiting for an answer from
that node (AWAIT) stops, with error 19, Message refused.  A GONE (type 5)
from a node's centre makes it forget the way to the node named (NO-ROUTE,
new), and ends a wait on it with error 20, Node gone.  From anyone else
it is ignored.  GONE ( n node -- ) sends one.

test_host_mesh.c, three nodes: text for a node nobody has a way to comes
back to the console, and to a node two away, as a NACK; a waiting sender's
line ends Message refused; a node that is waiting keeps what it has room
for, owes eight refusals, counts the rest, and sends them when its wait is
ended by a GONE from its centre.  70 checks, both widths, sanitizers.

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

v4/include/v4/message.h

@@ -33,6 +33,8 @@
 #define V4_MSG_TEXT   1                  /* text to be interpreted */
 #define V4_MSG_OUTPUT 2                  /* what a node printed */
 #define V4_MSG_DONE   3                  /* how the text ended: one word, V4_TEXT_* */
+#define V4_MSG_NACK   4                  /* a message was refused: one word, the node it was for (MESH.md 7b) */
+#define V4_MSG_GONE   5                  /* a node is gone: one word, its number */
 
 /* how text ended */
 #define V4_TEXT_QUIT      0              /* QUIT: nothing is to be said */