Add coverage comparison script

A script that takes two dirs containing coverage summaries for each app
and package, measured at two points (i.e. PR vs main) and fails if reg-
gression for any project is detected.

This script needs to be merged before a PR is created with a workflow
action referencing it, as that PR itself will trigger the job and fail
if the script is not found.

Author: douglaswinter

scripts/compare-coverage.mjs

@@ -0,0 +1,59 @@
+/**
+ * compare-coverage.mjs
+ *
+ * Compares Vitest coverage summary files for each app and package
+ * captured at two points, and fails if coverage has regressed.
+ *
+ * Intended for use in CI to prevent pull requests from decreasing test coverage.
+ *
+ * Arguments:
+ *   node compare-coverage.mjs <pr-dir> <baseline-dir>
+ *
+ * Exit codes:
+ *   0  coverage is unchanged or improved
+ *   1  coverage regressed in at least on project (or general script error)
+ */
+
+import fs from "node:fs";
+
+const prDir = process.argv[2];
+const baseDir = process.argv[3];
+
+const projects = fs.readdirSync(prDir).filter((file) => file.endsWith(".json"));
+
+let failed = false;
+
+for (const file of projects) {
+  const prPath = `${prDir}/${file}`;
+  const basePath = `${baseDir}/${file}`;
+
+  if (!fs.existsSync(basePath)) {
+    console.log(`⚠️ ${file.replace(".json", "")}: no baseline found, skipping`);
+    continue;
+  }
+
+  const pr = JSON.parse(fs.readFileSync(prPath));
+  const base = JSON.parse(fs.readFileSync(basePath));
+
+  const prLines = pr.total.lines.pct;
+  const baseLines = base.total.lines.pct;
+
+  const delta = (prLines - baseLines).toFixed(2);
+  const name = file.replace(".json", "");
+
+  console.log(`\nProject: ${name}`);
+  console.log(`Base: ${baseLines}%`);
+  console.log(`PR:   ${prLines}%`);
+  console.log(`Δ:    ${delta}%`);
+
+  if (prLines < baseLines) {
+    console.log("❌ Regression detected");
+    failed = true;
+  } else {
+    console.log("✅ OK");
+  }
+}
+
+if (failed) {
+  process.exit(1);
+}