docs(engine): clarify offline tick batching vs save-layer clamp

src/engine/save.ts

@@ -13,6 +13,10 @@ import { TICK_MS } from './tickLoop';
  * into a validated, versioned, compressed string and back. Saves carry a
  * `version` so future migrations have a hook; for now a version mismatch is
  * rejected cleanly rather than silently coerced.
+ *
+ * Offline credit: `applyOfflineProgress` floors elapsed ms to whole ticks
+ * (same `TICK_MS` as the live loop) and runs `tickGame` that many times.
+ * Elapsed beyond `DEFAULT_MAX_OFFLINE_MS` is not credited.
  */
 
 export const SAVE_VERSION = 1;