feat(v4.0.0): step 6e -- a node removed at any step leaves the rest at rest; written up

test_host_mesh.c, 220 checks: three exchanges on the row of three -- text
from the console for node 12 with what it prints and how it ended; node 11
SENDing node 12 text and waiting for its answer; a message node 12 has no
way for and its NACK to node 10 -- are each played once for every engine
step they take with node 11 removed at that step, and again with node 12:
17,642 removals at 64 bits.  Every node left comes to rest asleep, blocked
at no port and in no operation, and answers.  What is typed is a word
defined beforehand, so that the steps are the exchange's and not the
dictionary being searched.

Removed from the engine, nothing using them: v4_fabric_interrupt_error, a
device's pending hook, v4_node_words_since_look, V4_ERROR_INTERRUPTED.

MESH.md: 4.1's table has the addresses after the ports; 7a and 7b say what
of them still holds; 7b.7's limit is closed; step 6e is written up.
v4/README.md likewise.

make -C v4 test and sanitize at both widths, hosted-check, lint.  Three
bare-metal boots with the typed session and a loop printing 400 numbers:
logs/20261013-165124 (amd64), -165428 (aarch64), -165818 (riscv64).  POST
538 of 538, word_count=317, dict_hash=0x0951d7e02e251554 on all six, and
the three sessions are the same character for character.  The boots were
built with the working tree's uncommitted change to kernel/Makefile that
adds the ISA to the kernel's version line; nothing else differs from this
commit.

The plan's final review is not done yet.

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

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