test(labels): reproduce moving base-tip warning

test/labels-reconcile.test.sh

@@ -175,6 +175,78 @@ expect "an unreadable head version is silent — never nag on a guess" "" \
 expect "a bare head over an unreadable base still warns" yes \
   "$(release_shape_warning 41 2.0.0 "" | grep -qF '::warning::' && echo yes || echo no)"
 
+# The pure matrix above proves the warning predicate. These two fixtures drive
+# the sweep boundary that chooses WHICH base tree feeds it (#275): Forgejo and
+# GitHub both expose the PR's moving base tip beside its fixed merge base, and
+# using the former manufactures a downgrade after an intervening release.
+release_shape_ref_probe() { # $1 = phantom | bump
+  (
+    REPO=owner/repo
+    LABELS_CONF="$FIXTURE_CONF"
+    CEREMONY_FORGE=github
+    mode="$1"
+    refs="$RTMP/release-shape-$mode-refs"
+    : >"$refs"
+    case "$mode" in
+      phantom)
+        head_ver=1.3.0 base_tip_ver=1.4.0 merge_base_ver=1.3.0 ;;
+      bump)
+        head_ver=1.4.0 base_tip_ver=1.3.0 merge_base_ver=1.3.0 ;;
+      *) return 2 ;;
+    esac
+    # shellcheck disable=SC2317 # reached through the GitHub backend selected above
+    gh() {
+      if [ "$1" = label ] && [ "$2" = list ]; then
+        core_label_rows | cut -d'|' -f1
+        return 0
+      fi
+      if [ "$1" = pr ] && [ "$2" = list ]; then
+        printf '701\n'
+        return 0
+      fi
+      if [ "$1" = pr ] && [ "$2" = view ]; then
+        jq -n '{mergeable:"MERGEABLE",statusCheckRollup:[]}'
+        return 0
+      fi
+      if [ "$1" = issue ] && [ "$2" = edit ]; then return 0; fi
+      case "$(forge_stub_path "$*")" in
+        *'repos/owner/repo/pulls/701 --jq .requested_reviewers'*) return 0 ;;
+        *repos/owner/repo/pulls/701/reviews*) return 0 ;;
+        *repos/owner/repo/pulls/701)
+          jq -n '{draft:false,user:{login:"fixture-builder"},
+                  head:{sha:"head"},base:{sha:"base-tip"},merge_base:"merge-base",
+                  labels:[{name:"state:addressing"},{name:"blocked"}],
+                  requested_reviewers:[],created_at:"2026-08-31T20:00:00Z"}' ;;
+        *repos/owner/repo/commits/head*) printf '2026-08-31T20:00:00Z\n' ;;
+        *repos/owner/repo/contents/VERSION\?ref=head*)
+          printf 'head\n' >>"$refs"
+          printf '%s' "$head_ver" | base64 ;;
+        *repos/owner/repo/contents/VERSION\?ref=base-tip*)
+          printf 'base-tip\n' >>"$refs"
+          printf '%s' "$base_tip_ver" | base64 ;;
+        *repos/owner/repo/contents/VERSION\?ref=merge-base*)
+          printf 'merge-base\n' >>"$refs"
+          printf '%s' "$merge_base_ver" | base64 ;;
+        *) printf '[]\n' ;;
+      esac
+    }
+    main
+  )
+}
+
+phantom_shape="$(release_shape_ref_probe phantom)"
+expect "an unchanged branch cut before a later base release emits no release-shape warning" \
+  no "$(grep -q 'release-shaped' <<<"$phantom_shape" && echo yes || echo no)"
+expect "the unchanged-branch guard compares head with the PR merge base" \
+  $'head\nmerge-base' "$(cat "$RTMP/release-shape-phantom-refs")"
+
+bump_shape="$(release_shape_ref_probe bump)"
+expect "a genuine version bump relative to the merge base keeps the warning text" \
+  yes "$(grep -qF '#701 is release-shaped (version 1.3.0 -> 1.4.0 at its head)' \
+    <<<"$bump_shape" && echo yes || echo no)"
+expect "the genuine-bump guard also compares head with the PR merge base" \
+  $'head\nmerge-base' "$(cat "$RTMP/release-shape-bump-refs")"
+
 # -- drafts are building, whoever is requested --------------------------------
 DRAFT=true HEAD_SHA=head1 REQUESTED="" REVIEWS_JSON='[]'
 expect "draft PR is building" state:building "$(decide_state)"