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/tests/test_fabric.c

@@ -273,6 +273,14 @@ int main(void)
     CHECK(ND(c)->n.mem[OUT] == 2, "who gets its next: %ld", (long)ND(c)->n.mem[OUT]);
     CHECK(ND(a)->n.asking && ND(a)->n.request == 3, "and the writer waits with its third");
     CHECK(v4_fabric_unwire(&f, a, 0) && settle(100) == 0 && ND(a)->n.asking, "with the wire cut it goes on waiting");
+    v4_fabric_gone_error(&f, 18);
+    CHECK(settle(100) == 0 && ND(a)->n.asking && ND(a)->n.faults == 0, "and still does when the fabric has an error for it: a bare node cannot take one");
+    v4_node_error_attach(&ND(a)->n, OUT + 20);
+    v4_node_fault_attach(&ND(a)->n, 900);
+    CHECK(v4_fabric_step(&f) == 1 && !ND(a)->n.asking && ND(a)->n.fault_kind == V4_FAULT_RAISED && ND(a)->n.fault_addr == 18
+          && ND(a)->n.p == 900 + (v4_cell)V4_FAULT_RAISED && ND(a)->n.mem[OUT + 20] == 18,
+          "a node that can take an error is let go, and the error is raised on it: its P is its handler");
+    CHECK(!v4_node_port_gone(&ND(c)->n, 18), "a node that is not blocked at a port is not touched");
 
     /* ---- asleep, awake, and removed, while looping; and a node born while others run ---- */
     v4_fabric_init(&f, places, 4);