Write a WP-Cron job that reliably runs on a low-traffic site

I scheduled a daily digest email using `wp_schedule_event()` with the `daily` recurrence,
but it's missing days entirely, sometimes running two days late, and occasionally seems
to send the digest twice in the same day.

ENVIRONMENT: [this is a niche B2B site with maybe 20-30 visits per day, on standard
shared hosting, no server-level cron access is confirmed set up yet]

Fix this like a senior dev who understands WP-Cron's actual mechanics, in order:
1. Explain the core issue precisely: WP-Cron isn't a real cron daemon, it's a
   pseudo-cron that only checks for due events when a page loads (`wp-cron.php` fires
   via a request triggered by front-end traffic). On a site with 20-30 visits a day,
   there can be multi-hour or even multi-day gaps with zero requests, so scheduled
   events simply don't fire until the next visitor happens to load a page.
2. Recommend disabling the pseudo-cron trigger with `define('DISABLE_WP_CRON', true);`
   in `wp-config.php`, and replacing it with a real system cron job (or the host's
   control panel cron feature) hitting `wp-cron.php` via `wget` or `curl` on a fixed
   schedule (e.g. every 15 minutes), which guarantees WP-Cron actually gets a chance to
   run its due events regardless of visitor traffic.
3. Explain the double-send symptom: if two nearly-simultaneous requests both trigger
   `wp-cron.php` (a visitor and the new server cron overlapping, or a monitoring
   service also pinging the site), WordPress can start two overlapping cron runs before
   the first one finishes and clears the scheduled event, causing the digest to fire
   twice. Fix this with `wp_next_scheduled()` checked at the very top of the hook
   callback plus a short-lived transient lock (e.g. `get_transient('digest_running')`)
   released only after the mail send completes.
4. Verify no duplicate `wp_schedule_event()` registrations exist: check that the
   scheduling code runs on `wp_schedule_event` behind an `if ( ! wp_next_scheduled(...) )`
   guard on plugin activation, not on every page load, since re-scheduling the same
   hook repeatedly is a separate common cause of the same doubled-send symptom.
5. Note the edge case: after switching to a real server cron, confirm the site's own
   security setup (a firewall rule or security plugin) isn't blocking direct
   `wp-cron.php` requests from the server's own IP, since some hardening configs
   inadvertently block exactly the request this fix depends on.

Replace the schedule name, hook, and traffic pattern with your actual situation.