Flag a recipe change that will not ship without a version bump

When edge already holds the version, a PKGBUILD whose recipe changed
(comments and blank lines aside) builds nothing and publishes nothing.
Arch bumps pkgrel only when the built package changes, which is the
reviewer's call, so this is a warning annotation on the PKGBUILD and a
job summary line, not a failure. Comment-only and hook-only changes
stay silent.

.github/workflows/build-pr.yml

@@ -226,6 +226,18 @@ jobs:
           if [[ -n "$plan" && -z "$(sed -E 's/.*would build: *//' <<<"$plan" | tr -d '[:space:]')" ]]; then
             echo "==> ${{ matrix.package }} (${{ matrix.arch }}): edge already holds this version, nothing to build"
             echo "built=false" >> "$GITHUB_OUTPUT"
+            # Arch bumps pkgrel only when the built package changes, which is
+            # the reviewer's call. When the recipe itself changed (comments and
+            # blank lines aside), flag it on the PKGBUILD without failing: the
+            # change ships only once pkgver or pkgrel moves. HEAD is the base
+            # branch; the PR's package directories are overlaid on it.
+            recipe="pkgbuilds/${{ matrix.package }}/PKGBUILD"
+            recipe_lines() { grep -vE '^[[:space:]]*(#|$)' || true; }
+            if ! cmp -s <(git show "HEAD:$recipe" 2>/dev/null | recipe_lines) <(recipe_lines < "$recipe"); then
+              note="PKGBUILD changed, but edge already holds this version of ${{ matrix.package }}, so the change will not ship until pkgver or pkgrel changes. Bump pkgrel if it affects the built package."
+              echo "::warning file=$recipe,title=Change will not ship::$note"
+              echo "$note" >> "$GITHUB_STEP_SUMMARY"
+            fi
             exit 0
           fi
           bin/build --mirror edge --arch "${{ matrix.arch }}" --package "${{ matrix.package }}"