The launcher supervises the host instead of exec'ing it

Jochen: "I thought we did not want to run the host under a systemd/openrc/init
loop, but instead had our own host-init program?" -- and that was right. I had
moved the give-up logic out of unit files and left RESTART in them, with the
launcher exec'ing the host and disappearing. So init still decided when the
host came back, which is the arrangement 0061 exists to remove.

The launcher now stays and supervises: starts the host as a child, waits,
decides. Init is asked for one thing, run this at boot. There is an OpenRC
script beside the systemd unit now, four lines each, which is the point --
a second init is transcription rather than a port.

The cost of not exec'ing is signals. A supervisor that exits while its child
runs leaves the host to be killed rather than to stop, and an apply interrupted
that way is the half-configured machine this project is about. So SIGTERM is
trapped, passed down, and waited on.

Two bugs, both found by the tests rather than by review:

A clean exit was counted as a failure. The host exits cleanly to stand aside
for a new binary after an upgrade (0057), so a host that upgraded itself three
times rolled itself back having worked perfectly every time. The counter now
counts CONSECUTIVE FAILURES, incremented after the wait rather than before the
start.

And when rolling back I reset the counter file but not the variable, so the
next failure counted from the old value -- the rolled-back version got one
attempt instead of three.

Also: the host now clears the counter when it completes a reconcile, at the
same moment it records known-good and for the same reason. Without it the count
only climbs, and a node up for months rolls itself back on its third ordinary
restart -- a healthy machine undone by its own recovery.

One test expectation was tightened rather than fixed: "resets the counter after
rolling back" asserted exactly 0, which was true only under the old
count-before-start semantics. It now asserts the property -- below the limit --
since 1 is correct after a rollback plus one failure.

32 launcher tests, all confirmed to bite.

cmd/mesh-host/main.go

@@ -327,6 +327,13 @@ func runApply(ctx context.Context, opts options, d *declaration.Declaration, sou
 					"  a rollback would have nothing to return to.\n", version, err)
 		}
 	}
+	// And tell the launcher this start worked. Without it the counter only climbs, and a node
+	// that has been up for months rolls itself back on its third ordinary restart.
+	if err := upgrade.ClearAttempts(upgrade.AttemptsPath(opts.state)); err != nil {
+		fmt.Fprintf(os.Stderr,
+			"mesh-host: applied, but could not clear the start counter: %v\n"+
+				"  this node may roll itself back after a few more restarts.\n", err)
+	}
 
 	if opts.json {
 		return writeJSON(report)