Make ephemeral option policy configurable for durable cron

[chubes4]

Problem

Markdown Database Integration currently treats the exact cron option as unconditionally ephemeral in WP_Markdown_Write_Engine::EPHEMERAL_OPTION_NAMES.

That is correct for request-driven WordPress runtimes that cannot execute background work, but it prevents runtimes with an explicit durable scheduler from preserving WordPress cron events. wp_schedule_single_event() succeeds in the active runtime, then the cron option is omitted from canonical storage and the event disappears after reconstruction.

Reproduction

1. Boot MDI primary mode from canonical Markdown/JSON state.
2. Call wp_schedule_single_event( time() - 1, 'example_hook', array(), true ) and flush canonical writes.
3. Reconstruct the runtime from canonical state.
4. wp_next_scheduled( 'example_hook' ) returns false.

The downstream Cloudflare scheduled-Worker gate proved this with a real plugin callback and reconstructed runtime.

Proposed contract

Make the exact ephemeral option-name policy filterable while retaining the current defaults. A runtime with a durable scheduler can remove cron from the ephemeral-name list; transients, doing_cron, and all existing behavior remain unchanged by default.

Suggested filter shape:

$names = apply_filters( 'markdown_database_integration_ephemeral_option_names', self::EPHEMERAL_OPTION_NAMES );

The owning layer should validate/filter the resulting list and include deterministic tests for:

• unchanged default cron ephemerality;
• opting cron into canonical option persistence;
• reconstructed scheduled-event visibility;
• malformed filter output failing safely or reverting to defaults.

Downstream tracker

• Cloudflare WordPress runtime: Add a Cloudflare-hosted Playground runtime backend wp-codebox#1838
• Merged persistence foundation: Persist functional WordPress state in Cloudflare wp-codebox#1886