add the offscale line, and fix a set -e bug the test exposed

The menu is one function now rather than being written out twice — it is shown
again after ? prints the long answer — and option 1 carries your line: offscale is
just tailscale and headscale, with our own sugar on top.

The bug it surfaced is the more useful half. load_answers used

    [[ -z "${MACHINE_ROLE:-}" ]] && MACHINE_ROLE="$value"

as the last statement in a while-read loop body. When the variable is already set
the test is false, the compound returns non-zero, and as the final statement in a
loop body under `set -e` that ends the script. The failure is silent about its
cause: the trap prints "Step: unknown" and a line number inside the library,
before pre-flight has run.

It needed both conditions to appear — an answers file on disk AND the variables
already set in the environment — which is why every earlier test missed it and
running with env overrides hit it immediately. Written as if/then now, and the
other lib files scanned for the same shape.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

scripts/setup/machine-setup/lib/tailscale.sh

@@ -55,6 +55,20 @@ tailscale_help() {
   echo "     lock you out of the machine, so there is always a second way in."
 }
 
+# The menu itself, in a function because it is shown twice — once to ask, and
+# again after ? has printed the long answer, so the reader is not dropped back at
+# a bare prompt having forgotten what the options were.
+tailscale_network_menu() {
+  info "Which network should this machine join?"
+  echo ""
+  echo "     [1] set up your own network        (offscale)"
+  echo "         offscale is just tailscale and headscale, with our own sugar on top"
+  echo "     [2] use a network you already run  (headscale, offscale)"
+  echo "     [3] the easy route                 (tailscale.com)"
+  echo "     [?] what are tailscale, headscale and offscale?"
+  echo ""
+}
+
 # The long answer, printed when somebody types ?. Covers all three names,
 # because the menu offers all three and two of them are not words anyone outside
 # this project would know.