CI: auto-release a version tag when Renovate merges a dependency update

auto-release/next-version.ts

@@ -0,0 +1,43 @@
+// Pure release-version math for the Renovate auto-release (see renovate.yml → auto-release job,
+// README → CI/CD). Renovate stamps each commit with a `Release-Bump: <updateType>` trailer; the
+// workflow feeds those values here to pick the next `vX.Y.Z` tag. Kept side-effect-free and unit
+// tested (next-version.test.ts) — the git/tag/push side lives in the workflow shell.
+
+export type Bump = "major" | "minor" | "patch";
+
+// A dependency change is always at least a patch; only a real major/minor escalates.
+export function bumpFromUpdateType(updateType: string): Bump {
+  if (updateType === "major") return "major";
+  if (updateType === "minor") return "minor";
+  return "patch";
+}
+
+export function maxLevel(updateTypes: string[]): Bump {
+  let level: Bump = "patch";
+  for (const updateType of updateTypes) {
+    const bump = bumpFromUpdateType(updateType);
+    if (bump === "major") return "major";
+    if (bump === "minor") level = "minor";
+  }
+  return level;
+}
+
+// Pre-1.0 (major===0) shifts every level down one notch, so a dependency major only bumps the 0.x
+// minor and we never auto-cross into 1.0.0 — that stays a deliberate human milestone.
+export function nextVersion(latestTag: string, level: Bump): string {
+  const match = /^v(\d+)\.(\d+)\.(\d+)$/.exec(latestTag);
+  if (!match) throw new Error(`latest tag must be vX.Y.Z, got ${JSON.stringify(latestTag)}`);
+  const major = Number(match[1]);
+  const minor = Number(match[2]);
+  const patch = Number(match[3]);
+  const effective: Bump = major === 0 ? (level === "major" ? "minor" : "patch") : level;
+  if (effective === "major") return `v${major + 1}.0.0`;
+  if (effective === "minor") return `v${major}.${minor + 1}.0`;
+  return `v${major}.${minor}.${patch + 1}`;
+}
+
+// CLI: node auto-release/next-version.ts <latestTag> [updateType...] → prints the next tag.
+if (process.argv[1]?.endsWith("/next-version.ts")) {
+  const [, , latestTag, ...updateTypes] = process.argv;
+  process.stdout.write(nextVersion(latestTag ?? "", maxLevel(updateTypes)));
+}