Add editor plugin mounts to v2 shell

[scruffian]

What?

Adds PluginArea mounting to the v2 boot shell so registered plugin UI can render in both full app and single-page modes, mounts the editor Preferences modal in the v2 lazy editor path, and loads v2 lazy-editor settings/assets with a Site Editor context.

Why?

The v2 Site Editor shell already provides SlotFillProvider, but it did not mount PluginArea, so registerPlugin() render callbacks had no shell-level mount point outside the editor canvas. The lazy editor path also opened editor/preferences from the editor more menu without rendering the corresponding modal shell.

The lazy editor settings/assets requests also used the generic post editor context. That meant plugins enabled for the Site Editor could be missed when they enqueue assets or filter settings based on core/edit-site, the site-editor screen, the old site-editor.php admin hook, or global $pagenow.

How?

This adds a boot-local PluginArea wrapper with the same error-notice behavior used by the v1 Site Editor, mounts it in both boot roots, declares the @wordpress/plugins dependency and TypeScript reference, and renders PreferencesModal from the private editor APIs inside the lazy editor.

The boot canvas now renders lazy editor with context="site-editor". For that context, lazy editor requests /wp-block-editor/v1/settings and /wp-block-editor/v1/assets with context=site-editor, and the REST controller maps that to core/edit-site, the site-editor screen, site-editor.php hook suffix, and $pagenow during asset collection.

Testing Instructions

1. Run npm run lint:js -- packages/lazy-editor/src/hooks/use-editor-assets.tsx packages/lazy-editor/src/hooks/use-editor-settings.tsx packages/lazy-editor/src/components/editor/index.tsx packages/boot/src/components/canvas/index.tsx packages/boot/src/components/plugin-area/index.tsx packages/boot/src/components/root/index.tsx packages/boot/src/components/root/single-page.tsx.
2. Run ./node_modules/.bin/tsc -p packages/lazy-editor/tsconfig.json --noEmit --pretty false && ./node_modules/.bin/tsc -p packages/boot/tsconfig.json --noEmit --pretty false.
3. Run php -l lib/experimental/class-wp-rest-block-editor-settings-controller.php.
4. With the extensible Site Editor enabled, enable Create Block Theme or another plugin that registers editor SlotFill UI and enqueues for the Site Editor, then confirm its UI renders in the v2 editor.
5. Open the editor more menu in the v2 editor, choose Preferences, and confirm the Preferences modal opens.

Testing Instructions for Keyboard

When testing a registered plugin UI in the v2 shell, use the keyboard to tab to any plugin-provided controls and confirm they can be focused and operated.

Screenshots or screencast

N/A

Use of AI Tools

This PR was prepared with AI assistance from Codex. The resulting code and checks were reviewed before submission.

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[github-actions]

Size Change: +440 B (+0.01%)

Total Size: 8.6 MB

📦 View Changed

Filename	Size	Change
build/modules/boot/index.min.js	53 kB	+161 B (+0.3%)
build/modules/lazy-editor/index.min.js	14.5 kB	+279 B (+1.96%)

_{compressed-size-action}

[github-actions]

Flaky tests detected in c5a8daa.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/27680650741
📝 Reported issues:

• [Flaky Test] Should save the changes #78689 in /test/e2e/specs/editor/plugins/wp-editor-meta-box.spec.js
• [Flaky Test] shouldn't conflict with server-side autosave #78706 in /test/e2e/specs/editor/various/autosave.spec.js

[youknowriad]

I've been looking at this:

• the PreferencesModal is fixed.
• I'm not sure about the changes related to the editor settings and $pagenow, I think these changes break what the boot package is about and its architecture. The lazy-editor already tries to load the third-party scripts that target the editor (obviously some things may still be missed). But I'd prefer not to do anything here and assess issues first. We should not assume that boot corresponds to the site editor like before.

So I'm opening a small PR out of this just for the PluginArea changes.

[youknowriad]

Closing this one as I landed a simpler alternative for now.