Scan: add empty wp-build dashboard scaffold behind feature flag

[dhasilva]

Fixes #

Proposed changes

• Scaffold a brand-new projects/packages/scan/ package (composer automattic/jetpack-scan-page, textdomain jetpack-scan-page, PHP namespace Automattic\Jetpack\Scan_Page) with the boilerplate every Jetpack package needs (composer.json, package.json, tsconfig, eslint, phan, phpcs, AGENTS.md, README.md, CHANGELOG.md).
• Wire an empty routes/index/ wp-build route whose stage.tsx renders only <h1>Scan</h1> — just enough for designers/QA to load the page once the feature flag is flipped.
• Introduce the rsm_jetpack_ui_modernization_scan filter (default off). When the filter is off, Jetpack_Scan::initialize() early-returns: no admin menu is registered, no hooks fire, and the package contributes nothing to the existing Jetpack UI.
• When the filter is on, register a "Jetpack > Scan" wp-admin submenu, load the wp-build polyfills + generated build files, bridge wp-build's auto-generated enqueue/render functions to our ?page=jetpack-scan slug, and silence admin_notices on the page. Mirrors the bridge approach landed in Scan: port Calypso overview to wp-admin (Phases 0–8) #48458.
• Ship class-rest-controller.php as a no-op stub so Jetpack_Scan::register_rest_routes() stays callable. Real routes land in the follow-up Scan: port Calypso overview to wp-admin (Phases 0–8) #48458 rebase.
• Wire the new package into projects/plugins/jetpack/ via composer.json + a Scan_Page_Init::initialize() call in class.jetpack.php (alongside Activity_Log_Init).

Related product discussion/links

• Mirrors the VideoPress equivalent: VideoPress: add empty wp-build dashboard scaffold behind feature flag #48494
• Source PR (will rebase on top of this scaffold): Scan: port Calypso overview to wp-admin (Phases 0–8) #48458

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

No. The package contributes nothing when the feature flag is off (the default), and the only thing it adds when flipped on is an empty admin page; no new tracking/data collection.

Testing instructions

With the filter off (default — confirms no regression):

• Check out this branch and run pnpm install && composer install in projects/plugins/jetpack/.
• Activate Jetpack and visit wp-admin.
• Confirm there is no "Scan" entry under the Jetpack menu and the rest of the Jetpack admin UI is unchanged.

With the filter on (confirms the wp-build scaffold renders):

• Drop the following in a mu-plugin:

  add_filter( 'rsm_jetpack_ui_modernization_scan', '__return_true' );

• Build the package: pnpm --filter @automattic/jetpack-scan-page run build.
• Reload wp-admin. A new "Scan" submenu should appear under Jetpack.
• Click it. The page should render a <h1>Scan</h1> placeholder served by wp-build (view-source should show the wp-build-generated <div id="wp-build-page-jetpack-scan-wp-admin"> mount + boot script).
• Confirm ?page=jetpack-scan does not 500 and no JS console errors.

[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 update/scan-modernization-phase-0 branch.
• To test on Simple, run the following command on your sandbox:

bin/jetpack-downloader test jetpack update/scan-modernization-phase-0

bin/jetpack-downloader test jetpack-mu-wpcom-plugin update/scan-modernization-phase-0

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:

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Code Coverage Summary

Coverage changed in 1 file.

File	Coverage	Δ%	Δ Uncovered
projects/plugins/jetpack/class.jetpack.php	748/2276 (32.86%)	-0.01%	1 ❤️‍🩹

Full summary · PHP report · JS report