Restore the novel script's lost module-scope constants (#74)

novel-bookmark.user.js was split out of manga-bookmark.user.js and lost
four constants: CACHE_KEY, LASTCHECKED_KEY, LATEST_CHECK_THROTTLE_MS and
LATEST_CHECK_BATCH. Every use sits behind a try/catch or a
fire-and-forget promise, so the ReferenceErrors were swallowed:
backgroundRefreshLatest() threw before its first fetch, and the local
cache silently read empty and never persisted.

The issue named the two throttle constants; the cache keys are the same
lost lines and are fixed here too.

Add a static guard over both scripts, since no behavioural test can see
a swallowed ReferenceError.

userscript/novel-bookmark.user.js

@@ -25,6 +25,13 @@
   // This script owns the novel library; the manga script is a separate install
   // with its own prefix, so the two never share a cache, a queue or a panel.
   const STORE_PREFIX = "bmgr:novel:";
+  const CACHE_KEY = STORE_PREFIX + "cache";
+
+  // Per-device record of when each series was last checked for new chapters.
+  // Deliberately not synced: each device does its own checking.
+  const LASTCHECKED_KEY = STORE_PREFIX + "lastchecked";
+  const LATEST_CHECK_THROTTLE_MS = 4 * 60 * 60 * 1000;
+  const LATEST_CHECK_BATCH = 1; // series fetched per navigation // series fetched per navigation
 
   // Which library this script's rows belong to. The manga script is a separate
   // install that declares "manga"; the backend keeps whichever it is told.