feedback5 R6: add a small checkJs slice and core typedefs

tsc checks shared (except protocol.js and bandBonds.js), the sim rng/dir/constants helpers, and types/core.js. strict stays off. types/README.md says how to widen the slice.

package.json

@@ -35,7 +35,8 @@
     "setup": "node tools/setup.mjs",
     "doctor": "node tools/doctor.mjs",
     "launch": "node scripts/launch.mjs",
-    "lint": "eslint ."
+    "lint": "eslint .",
+    "typecheck": "tsc --noEmit --checkJs -p jsconfig.json"
   },
   "engines": {
     "node": ">=22"
@@ -52,6 +53,7 @@
     "@eslint/js": "^10.0.1",
     "eslint": "^10.12.0",
     "globals": "^17.13.0",
-    "puppeteer-core": "^25.12.0"
+    "puppeteer-core": "^25.12.0",
+    "typescript": "^5.9.2"
   }
 }