feat(core-eslint): component-must-have-test rule

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/core-eslint/rules/component-must-have-test.js

@@ -0,0 +1,53 @@
+import fs from "node:fs";
+import path from "node:path";
+
+function isComponentFile(filename) {
+  if (!filename.endsWith(".tsx")) return false;
+  const base = path.basename(filename);
+  if (
+    base === "index.tsx" ||
+    base.endsWith(".test.tsx") ||
+    base.endsWith(".stories.tsx") ||
+    base.endsWith(".spec.tsx")
+  ) {
+    return false;
+  }
+  return (
+    filename.includes("/packages/core-ui/src/") ||
+    /\/packages\/[^/]+\/src\/ui\//.test(filename)
+  );
+}
+
+/** @type {import("eslint").Rule.RuleModule} */
+export default {
+  meta: {
+    type: "problem",
+    docs: {
+      description:
+        "Every component file must have a sibling *.test.tsx for behavioural coverage.",
+    },
+    schema: [],
+    messages: {
+      missingTest:
+        "Component {{filename}} has no sibling test at {{expected}}. Write the red test first.",
+    },
+  },
+  create(context) {
+    return {
+      Program(node) {
+        const filename = context.filename;
+        if (!isComponentFile(filename)) return;
+        const expected = filename.replace(/\.tsx$/, ".test.tsx");
+        if (fs.existsSync(expected)) return;
+        context.report({
+          node,
+          messageId: "missingTest",
+          data: {
+            filename: path.basename(filename),
+            expected: path.basename(expected),
+          },
+        });
+      },
+    };
+  },
+};