ci: release a patch version automatically when the map changes

Every master build fingerprints the rendered map (PNG without the revision line) and compares it with the latest release. A changed map publishes the next patch release with the PDF and PNG; otherwise the site keeps showing the latest version. Minor and major releases are started manually from the workflow. Versions restart at v2.0.0.

.github/scripts/next-version.sh

@@ -0,0 +1,47 @@
+#!/usr/bin/env bash
+# Decides the roadmap version for a build.
+# - Patch releases are automatic: a new one is due when the rendered map differs from the latest release's.
+# - Minor and major releases only happen when a maintainer asks for them (BUMP=minor|major).
+# Inputs: FINGERPRINT (sha256 of the map PNG rendered without the revision line), BUMP (auto|minor|major),
+#         GH_REPO and GH_TOKEN for the GitHub API. Outputs (to $GITHUB_OUTPUT): release, version, previous, date.
+set -euo pipefail
+: "${FINGERPRINT:?}" "${GH_REPO:?}"
+BUMP="${BUMP:-auto}"
+
+tag=v0.0.0 published="" body=""
+if info="$(gh api "repos/$GH_REPO/releases/latest" --jq '[.tag_name, .published_at[0:10]] | @tsv' 2>/dev/null)"; then
+  IFS=$'\t' read -r tag published <<<"$info"
+  body="$(gh api "repos/$GH_REPO/releases/latest" --jq '.body // ""')"
+fi
+previous_fingerprint="$(grep -oE 'map-fingerprint: [0-9a-f]{64}' <<<"$body" | cut -d' ' -f2 || true)"
+
+if [[ ! $tag =~ ^v([0-9]+)\.([0-9]+)\.([0-9]+)$ ]]; then
+  echo "::error::The latest release tag '$tag' is not in vMAJOR.MINOR.PATCH form."
+  exit 1
+fi
+major=${BASH_REMATCH[1]} minor=${BASH_REMATCH[2]} patch=${BASH_REMATCH[3]}
+
+release=true
+if (( major < 2 )); then
+  # Versions restart at 2.0.0 with the roadmap rendered from the site.
+  version=v2.0.0
+elif [[ $BUMP == major ]]; then
+  version="v$((major + 1)).0.0"
+elif [[ $BUMP == minor ]]; then
+  version="v$major.$((minor + 1)).0"
+elif [[ $FINGERPRINT != "$previous_fingerprint" ]]; then
+  version="v$major.$minor.$((patch + 1))"
+else
+  release=false
+  version=$tag
+fi
+date="$([[ $release == true ]] && date -u +%F || echo "$published")"
+
+echo "Latest release $tag (map ${previous_fingerprint:-unknown}); this map $FINGERPRINT; bump $BUMP"
+echo "=> release=$release version=$version date=$date"
+{
+  echo "release=$release"
+  echo "version=$version"
+  echo "previous=$tag"
+  echo "date=$date"
+} >> "${GITHUB_OUTPUT:-/dev/stdout}"