add the engine benchmark: make, sh, just and a Go prototype

Same work on each, median of five. The numbers separate three costs that the
earlier measurements mixed up: the engine itself (go 23 ms flat, sh 177 ms
plus 63 per stack, just 204 plus 32, make 792 plus 700), the shell hooks
(about 40 ms per computed setting whatever the engine, since Go runs the same
sh), and bin/myos loading a directory's hooks once per stack reference rather
than once per directory, which doubles the hook cost for a group.

Memoising the lazy defaults changes nothing: the cost is the command
substitutions inside each tag helper, not repeated lookups.

spec/bench/run.sh

@@ -0,0 +1,39 @@
+#!/bin/sh
+# the full matrix: 5 runs each, median, every engine on the same work
+. /tmp/myos-bench/env.sh; cd "$WORKDIR"
+B=/tmp/myos-bench/bench.sh; N=5
+MK="make -esC $MYOS_ROOT MYOS=. WORKDIR=$WORKDIR"
+SH="$MYOS_ROOT/bin/myos"
+JU="just --justfile /tmp/myos-bench/justfile"
+GO=/tmp/myos-bench/myos-go
+S1="host/consul"; S2="host/consul host/fabio"; S3="host/consul host/fabio host/registrator"
+
+echo "== cout fixe : demarrage + cible vide"
+$B "make  noop" $N -- $MK FORCE
+$B "sh    noop (myos version)" $N -- $SH version
+$B "just  noop (parse + 1 sh + source lib/)" $N -- $JU noop
+$B "go    noop" $N -- $GO noop
+echo
+echo "== up : 1 / 2 / 3 stacks, dry-run"
+$B "make  up 1" $N -- $MK up STACK="$S1"
+$B "make  up 2" $N -- $MK up STACK="$S2"
+$B "make  up 3" $N -- $MK up STACK="$S3"
+$B "sh    up 1" $N -- $SH up host/consul
+$B "sh    up 2" $N -- $SH up host/consul host/fabio
+$B "sh    up 3" $N -- $SH up host/consul host/fabio host/registrator
+$B "just  up 1" $N -- $JU up host/consul
+$B "just  up 2" $N -- $JU up host/consul host/fabio
+$B "just  up 3" $N -- $JU up host/consul host/fabio host/registrator
+$B "go    up 1" $N -- $GO up host/consul
+$B "go    up 2" $N -- $GO up host/consul host/fabio
+$B "go    up 3" $N -- $GO up host/consul host/fabio host/registrator
+echo
+echo "== export : les 80 reglages du groupe host (evaluation des hooks shell)"
+$B "sh    export, hooks tels quels" $N -- $SH export STACK=host
+$B "just  export, hooks tels quels" $N -- $JU export host
+$B "go    export, hooks tels quels (1 sh/repertoire)" $N -- $GO export host
+echo
+echo "== export : memes hooks, evalues en une passe (MYOS_VAR_MEMO=1)"
+MYOS_VAR_MEMO=1 $B "sh    export, memoise" $N -- $SH export STACK=host
+MYOS_VAR_MEMO=1 $B "just  export, memoise" $N -- $JU export host
+MYOS_VAR_MEMO=1 $B "go    export, memoise" $N -- $GO export host