Podcast: simplify wp-build wiring per #48557 review

[arcangelini]

Follow-up to #48557 and #48559 addressing review feedback from @simison.

Proposed changes

• Slim Admin_Page (projects/packages/podcast/src/class-admin-page.php) down to the same wp-build wiring shape as Jetpack_Backup (Backup: add empty wp-build dashboard scaffold behind feature flag #48583): polyfills register + wp-build entry require + current_screen alias of $screen->id to jetpack-podcast-dashboard.
• Drop bridge_wp_build_enqueue() (the $_GET[page] swap) and fix_boot_import_map_ordering() (the boot-script reorder workaround). wp-build's auto-generated enqueue callback handles enqueue once $screen->id matches; the import-map fix isn't needed in this path.
• Collapse the Podcast slug in wpcom_reorder_submenu() from two entries (jetpack-podcast, podcasting) to a single podcast. wpcom_reorder_submenu does substring matching, and only one of the two URLs is ever registered at a time, so one entry covers both flag states.
• Render the dashboard Stage inside Page from @wordpress/admin-ui (matching the Newsletter pattern in Newsletter: port Subscribers DataViews onto the modernization chassis #48581) — title/subtitle in the page header, tabs in the body. Replaces the bare <h1>Podcast</h1> + <Tabs.Root> layout.

Related product discussion/links

n/a

Does this pull request change what data or activity we track or use?

No.

Testing instructions

1. Pull this branch.
2. pnpm jetpack build packages/podcast --deps.
3. Add the filter to a mu-plugin / wp-config.php:

   add_filter( 'jetpack_podcast_untangle', '__return_true' );

4. Visit wp-admin/admin.php?page=jetpack-podcast. The page should render inside the @wordpress/admin-ui Page chrome — "Podcast" title, the tagline as subtitle, and the four placeholder tabs (Settings / Episodes / Distribution / Stats) below. DevTools → Network should request from build/pages/jetpack-podcast-dashboard/. No console errors.
5. Without the build artifact (delete projects/packages/podcast/build/ and reload), the page still renders the bare <h1>Podcast</h1> placeholder via the PHP fallback Admin_Page::render().
6. Disable the filter and confirm the legacy "Jetpack > Podcasting" Calypso link is back exactly as before.
7. With the filter on, confirm the "Jetpack > Podcast" entry appears in the position previously held by "Jetpack > Podcasting" in the Jetpack submenu (between Newsletter and Traffic).

[github-actions]

Are you an Automattician? Please test your changes on all WordPress.com environments to help mitigate accidental explosions.

• To test on WoA, go to the Plugins menu on a WoA dev site. Click on the "Upload" button and follow the upgrade flow to be able to upload, install, and activate the Jetpack Beta plugin. Once the plugin is active, go to Jetpack > Jetpack Beta, select your plugin (Jetpack or WordPress.com Site Helper), and enable the arcangelini/podcast-make-it-better branch.
• To test on Simple, run the following command on your sandbox:

bin/jetpack-downloader test jetpack arcangelini/podcast-make-it-better

bin/jetpack-downloader test jetpack-mu-wpcom-plugin arcangelini/podcast-make-it-better

Interested in more tips and information?

• In your local development environment, use the jetpack rsync command to sync your changes to a WoA dev blog.
• Read more about our development workflow here: PCYsg-eg0-p2
• Figure out when your changes will be shipped to customers here: PCYsg-eg5-p2

[github-actions]

Thank you for your PR!

When contributing to Jetpack, we have a few suggestions that can help us test and review your patch:

• ✅ Include a description of your PR changes.
  
• ✅ Add a "[Status]" label (In Progress, Needs Review, ...).
  
• ✅ Add testing instructions.
  
• ✅ Specify whether this PR includes any changes to data or privacy.
  
• ✅ Add changelog entries to affected projects
  

This comment will be updated as you work on your PR and make changes. If you think that some of those checks are not needed for your PR, please explain why you think so. Thanks for cooperation 🤖

---

Follow this PR Review Process:

1. Ensure all required checks appearing at the bottom of this PR are passing.
2. Make sure to test your changes on all platforms that it applies to. You're responsible for the quality of the code you ship.
3. You can use GitHub's Reviewers functionality to request a review.
4. When it's reviewed and merged, you will be pinged in Slack to deploy the changes to WordPress.com simple once the build is done.

If you have questions about anything, reach out in #jetpack-developers for guidance!

[jp-launch-control]

Code Coverage Summary

This PR did not change code coverage!

That could be good or bad, depending on the situation. Everything covered before, and still is? Great! Nothing was covered before? Not so great. 🤷

Full summary · PHP report · JS report