fix(v4.0.0): a node blocked at a port with no one on it is given an error, not left there

A node writing to a node that was stuck, and was then killed, stayed
blocked for ever: killing a stuck node could cost its neighbours.  And on
the hosted and bare-metal products a write to an empty port, 5 7 PORT!,
ended the program with "the node stopped".

Now a node that can take an error, blocked writing to or reading from one
port that nothing is wired to -- nothing ever was, or what was there has
been killed or the wire cut -- has error 18, No one on that port, raised
on it and goes on.  A bare node waits as on the fabric; so does a node
whose neighbour is asleep, and one reading "any port".

test_host_unit.c: Hera kills node 14 while node 12 is blocked writing to
it.  It failed first: 12 stayed blocked.  test_fabric.c: a bare node still
waits.  hosted-check types 5 7 PORT! and goes on; it failed first too.

make -C v4 test, sanitize and hosted-check pass; amd64, aarch64 and
riscv64 boot, POST 538 of 538, dict_hash 0x5f0a949a6fc8ef2b on all six:
logs/20261007-121743, -122008, -122337.

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

v4/src/node.c

@@ -222,3 +222,13 @@ void v4_node_port_served(v4_node *n)
 {
     n->asking = 0;
 }
+
+int v4_node_port_gone(v4_node *n, v4_cell code)
+{
+    if (code == 0 || n->error_reg < 0 || n->fault_vector < 0 || n->stopped) return 0;
+    if (n->asking) n->asking = 0;
+    else if (n->reading && !n->given && n->read_port != V4_PORT_ANY) n->reading = 0;
+    else return 0;
+    v4_node_store(n, n->error_reg, code);            /* raises it: P is the node's handler now */
+    return 1;
+}