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/include/v4/node.h

@@ -214,6 +214,16 @@ void v4_node_port_attach(v4_node *n, v4_cell base);
 /* The neighbour has taken what the node wrote. */
 void v4_node_port_served(v4_node *n);
 
+/* THERE IS NO ONE ON THE PORT the node is blocked at -- writing to it, or
+ * reading from that one port -- and there will not be.  A node that can
+ * take an error (it has an error register and a fault table: node.h,
+ * v4_node_error_attach, v4_node_fault_attach) is let go and the error
+ * `code` is raised on it, as if it had stored the code to its error
+ * register: what it was doing ends there.  Returns 1.  A node that cannot
+ * stays blocked, as on the fabric, and 0 is returned; so is 0 for a node
+ * that is not blocked at a port, or is reading "any port". */
+int v4_node_port_gone(v4_node *n, v4_cell code);
+
 /* The neighbour on `port` writes `value`: the node, blocked reading that
  * port or "any", is unblocked and will fetch it. */
 void v4_node_port_give(v4_node *n, unsigned port, v4_cell value);