feat(core-eslint): manifest source helper for conformance rules

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

packages/core-eslint/rules/_manifest-source.js

@@ -0,0 +1,57 @@
+import fs from "node:fs";
+import path from "node:path";
+
+/**
+ * Reads a feature.manifest.ts file and extracts the manifest's `name` field
+ * and `requiredCores` array via regex. Returns null if the file does not
+ * exist or does not match the expected literal `as const` manifest shape.
+ *
+ * The repo's convention is that every feature.manifest.ts uses defineFeature
+ * with literal `as const` syntax — this is enforced by the type-system
+ * design (defineFeature has `<const M>` to preserve literal types). The
+ * regex extraction is therefore safe; the AST path is overkill.
+ *
+ * Returns: { name: string, requiredCores: string[] } | null
+ */
+export function readManifestSource(manifestPath) {
+  let src;
+  try {
+    src = fs.readFileSync(manifestPath, "utf8");
+  } catch {
+    return null;
+  }
+  const nameMatch = src.match(/name:\s*"([^"]+)"/);
+  if (!nameMatch) return null;
+  const coresMatch = src.match(/requiredCores:\s*\[([^\]]*)\]/);
+  const cores = coresMatch
+    ? coresMatch[1]
+        .split(",")
+        .map((s) => s.trim().replace(/^"/, "").replace(/"$/, ""))
+        .filter((s) => s.length > 0)
+    : [];
+  return { name: nameMatch[1], requiredCores: cores };
+}
+
+/**
+ * Canonical manifest path for a given feature package root.
+ */
+export function manifestPathForFeature(featureRoot) {
+  return path.join(featureRoot, "src", "feature.manifest.ts");
+}
+
+/**
+ * Given an absolute file path and the monorepo root, returns the feature
+ * package root that contains the file (e.g. /repo/packages/auth). Returns
+ * null when the file lives outside the packages/ tree.
+ *
+ * Assumes the conventional layout `packages/<feature>/src/...`.
+ */
+export function featureRootForFile(filepath, repoRoot) {
+  const packagesDir = path.join(repoRoot, "packages");
+  if (!filepath.startsWith(packagesDir + path.sep)) return null;
+  const rel = filepath.slice(packagesDir.length + 1); // e.g. "auth/src/..."
+  const slash = rel.indexOf(path.sep);
+  if (slash === -1) return null;
+  const featureName = rel.slice(0, slash);
+  return path.join(packagesDir, featureName);
+}