Fix updater tests for image pinning; harden helper load (node-26#19)

- test_host_updater: stub git/docker now model cat-file/show/rev-parse
  --verify and compose config/image inspect. The two update-path tests
  were failing on Linux CI (skipped locally on macOS, no flock). Adds
  tests for pull-before-checkout + .env pins, a missing pinned image
  leaving the node unchanged, and a pre-pinning target still updating.
- drb-update.sh: load image-tags.sh via a variable + eval and confirm the
  functions exist; `source <(git show ...)` "succeeded" with nothing
  defined when git show failed, wedging every later update.
- install.sh: skip pinning (warn) when the checked-out ref predates
  scripts/image-tags.sh instead of dying on the missing file.

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

scripts/drb-update.sh

@@ -158,10 +158,12 @@ do_update() {
   # the git tree hash of its own source directory, so the images always match
   # the code being checked out. The helper is read from the target commit so
   # its service list matches the target's compose file.
-  local pin=0
+  # Read into a variable, not `source <(...)`: a failed `git show` must leave
+  # pin=0 rather than "succeed" with no functions defined.
+  local pin=0 helper
   if git cat-file -e "$new:scripts/image-tags.sh" 2>/dev/null; then
-    # shellcheck disable=SC1090
-    source <(git show "$new:scripts/image-tags.sh") && pin=1
+    helper="$(git show "$new:scripts/image-tags.sh")" && [ -n "$helper" ] && eval "$helper" \
+      && declare -F drb_image_tags drb_images_present drb_pin_image_tags >/dev/null && pin=1
   fi
   [ "$pin" = 1 ] || log "target ${new:0:12} predates image pinning — using its compose file's default tags"