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>

docs/v4.0.0/MESH.md

@@ -116,6 +116,29 @@ word address `base`:
   That is how an F18 node runs code from a port, and it is what makes
   ruling 8 need no code in a newborn node.
 - A port with nothing on the other end blocks for ever, as on the fabric.
+  **Fixed 2026-10-07 ("Fix the error. no bad code is ever released"):**
+  that holds for a bare node. A node that can take an error is not left
+  there. When it is blocked writing to one port, or reading from one, that
+  nothing is wired to — because nothing ever was, or because what was
+  there has been killed or the wire cut — error 18, "No one on that
+  port", is raised on it: what it was doing ends with the message and it
+  goes on (`v4/src/node.c`, `v4_node_port_gone`; `v4/src/fabric.c`,
+  `v4_fabric_gone_error`; the lone node, `v4/system/boot.c`). A node whose
+  neighbour is asleep waits, and so does one reading "any port".
+  What it mended: a node writing to a node that was stuck, and was then
+  killed, stayed blocked for ever, so that killing a stuck node could cost
+  its neighbours (found while step 7 was being designed); and on the two
+  products a write to an empty port, `5 7 PORT!`, ended the program.
+  `v4/tests/test_host_unit.c`: Hera kills node 14 while node 12 is blocked
+  writing to it; 12's line ends "No one on that port", every other node
+  answers, and a later send to 14 is that error at once.
+  `v4/tests/test_fabric.c`: a bare node still waits. `hosted-check` and the
+  three boots type `5 7 PORT!` and go on: `logs/20261007-121743` (amd64),
+  `-122008` (aarch64), `-122337` (riscv64); POST 538 of 538,
+  `dict_hash=0x5f0a949a6fc8ef2b` on all six.
+  **Not mended, and reported:** a node that waits at "any port" for an
+  answer that never comes (`AWAIT`, on a node that never answers) waits
+  for ever, and on the lone node that ends the line as "stopped".
 
 ### 4.2 A node at reset