feat: type the plugin host record, move plugin strings out of core and add lint checks

scripts/check-test-locations.cjs

@@ -0,0 +1,61 @@
+#!/usr/bin/env node
+/**
+ * Tests mirror the source tree in their own directories and never sit next
+ * to the code they test. A *.test.ts(x) file may only live under
+ * src/backend/tests/, src/ui/tests/, scripts/ or plugins/<id>/tests/.
+ */
+
+const fs = require("node:fs");
+const path = require("node:path");
+
+const ROOT = path.resolve(__dirname, "..");
+const SKIP = new Set([
+  "node_modules",
+  ".git",
+  "dist",
+  "build",
+  "release",
+  "coverage",
+  ".vite",
+  ".turbo",
+]);
+const TEST_FILE = /\.test\.tsx?$/;
+const ALLOWED = [
+  /^src\/backend\/tests\//,
+  /^src\/ui\/tests\//,
+  /^scripts\//,
+  /^plugins\/[^/]+\/tests\//,
+];
+
+function walk(dir, root, out) {
+  for (const entry of fs.readdirSync(dir, { withFileTypes: true })) {
+    if (SKIP.has(entry.name)) continue;
+    const full = path.join(dir, entry.name);
+    if (entry.isDirectory()) walk(full, root, out);
+    else if (TEST_FILE.test(entry.name)) {
+      out.push(path.relative(root, full).replaceAll("\\", "/"));
+    }
+  }
+  return out;
+}
+
+/** Test files outside the directories tests belong in. */
+function misplaced(root = ROOT) {
+  return walk(root, root, [])
+    .filter((file) => !ALLOWED.some((pattern) => pattern.test(file)))
+    .sort();
+}
+
+function main() {
+  const found = misplaced();
+  if (found.length === 0) return;
+  console.error(
+    "Tests belong in src/backend/tests/, src/ui/tests/, scripts/ or plugins/<id>/tests/, mirroring the source:",
+  );
+  for (const file of found) console.error(`  ${file}`);
+  process.exit(1);
+}
+
+if (require.main === module) main();
+
+module.exports = { misplaced };