Build the rollback mechanism, and test it

ADR 0059's recovery path: the pieces that run when the host will not start.

internal/upgrade -- two facts, neither of them the host judging its health.
Whether the executable this process started from has been replaced on disk, and
which version last completed a reconcile.

The first design was wrong and the tests caught it, not review. It asked
/proc/self/exe whether it was marked deleted. That is Linux procfs behaviour
rather than a fact about files, and it catches only unlink -- a binary swapped
by rename onto the same path reads as untouched, which is exactly what a
package manager does. Now the identity is captured at start and compared later:
no procfs, and neither case missed.

known-good is one bare line. The reader is a shell script on a machine where
the host is failing to start, so it must not need a parser to be present and
working. Written only after a clean apply, which is the whole claim -- not
health, because a disconnected node is ordinary and a failing resource is the
machine's problem rather than the binary's.

packaging/ -- the unit, the rollback unit, and the rollback script. The script
shares no code with the host and calls none of it: a binary that cannot start
cannot be its own recovery. POSIX sh, nothing that has to be installed. The
unit carries Restart=always with a comment saying why on-failure would break
every upgrade.

Both are tested and both sets of tests were confirmed to bite. Injecting five
faults broke exactly the intended tests -- except one, and chasing why it did
not found a placebo assertion I had written: `check "exits zero" ... "0" "0"`
compares a literal to itself and can never fail. Replaced with the real exit
code, after which the injection bites.

Also caught: an injection that produced a build failure rather than a test
failure, which my grep read as "no failure". Re-run so it compiled, and the
test did bite.

The script test runs in `make check`, so it is a gate rather than something
that was run once.

Verified against the real binary: known-good is written beside the store after
a clean apply and is NOT written after a failed one.

packaging/nox-mesh-host.service

@@ -0,0 +1,21 @@
+[Unit]
+Description=Novox Mesh node host
+After=network-online.target
+Wants=network-online.target
+# When the supervisor gives up, recover rather than leaving the node quiet — a host that will
+# not start looks exactly like a machine somebody switched off (novox/hq ADR 0059).
+OnFailure=nox-mesh-host-rollback.service
+
+[Service]
+Type=notify
+ExecStart=/usr/bin/nox-mesh-host run
+# always, NOT on-failure: the host restarts onto a new binary by exiting CLEANLY
+# (novox/hq ADR 0057), and on-failure would leave an upgraded node stopped.
+Restart=always
+RestartSec=5s
+StartLimitBurst=3
+StartLimitIntervalSec=120
+StateDirectory=mesh-host
+
+[Install]
+WantedBy=multi-user.target