[WIP] Add first-class stateful iterator to Cron class

[Copilot]

Thanks for asking me to work on this. I will get started on it and keep this PR's description up to date as I form a plan and make progress.

Add a first-class, ECMAScript-compliant stateful iterator to the Cron class to allow sequential date traversal without breaking the existing stateless API.
​Technical Requirements:
​Factory Method: Add Cron.prototype.enumerate(startAt?) to the main class. It should return a new CronIterator instance.
​CronIterator Class: * Maintain an internal cursor initialized to startAt or Date.now().
​Implement the Iterator Protocol: .next() must return { value: Date, done: boolean }.
​Implement the Iterable Protocol: Symbol.iterator { return this; } to enable for...of loops and destructuring.
​Logic: Each .next() call should advance the cursor using the parent Cron instance’s next(this.cursor) logic.
​Boundary Handling: If the schedule reaches a defined stopAt or the end of a sequence, return done: true.
​Utility Methods:
​.peek(): Return the next date without advancing the cursor.
​.reset(newStartAt?): Reset the internal cursor.
​Success Criteria:
​for (const date of job.enumerate()) works natively.
​const [a, b] = job.enumerate() correctly captures the next two occurrences.
​The core Cron class remains stateless (calling enumerate does not change the behavior of the base instance).

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