Skip async CSS on Canvas editor and other editorial overlays that use the front-end theme

Problem/Motivation

The Canvas backend editor (/canvas/editor/…) uses the site default (front-end) theme. This module enables async remaining CSS whenever the active theme is in enabled_themes / the theme’s info.yml. The only extra skip is media.oembed_iframe. There is no equivalent of SDC Critical CSS’s editorial skip (_admin_route, _hide_admin_toolbar, iframe).

Result: Canvas editor UI CSS is loaded as preload-as-style. Styles break on the edit screen. The same sites are fine with SDC Critical CSS alone on public Canvas pages.

Mercury Editor preview iframes can hit the same path (front-end theme + async CSS). Companion issue on SDC Critical CSS covers inlining/aggregate stripping there.

Steps to reproduce

1. Enable Non-critical CSS for the default theme (and SDC Critical CSS as documented).
2. Open a Canvas editor URL, e.g. /canvas/editor/canvas_page/{id}.
3. Editor styles are broken. Public pages of the same site are OK.

Observed on Drupal CMS Canvas site templates (e.g. Convene).

Proposed resolution

Add a skip in shouldOptimizeCssDelivery() (or equivalent) before async rendering. Mirror SDC Critical CSS:

• _admin_route
• _hide_admin_toolbar
• Sec-Fetch-Dest: iframe
• _mercury_editor_route
• route name matching mercury_editor.* / *.mercury_editor_preview / canvas.*
• path prefix /mercury-editor or /canvas/

No hard dependency on Canvas or Mercury. Optional alter hook later if other editors need an in. Keep the oEmbed skip.

Remaining tasks

• Implement the skip; do not async CSS on those requests.
• Tests for Canvas editor path, admin route, iframe, oEmbed still skipped.
• README: editorial overlays are skipped; public pages unchanged.

User interface changes

None.

API changes

None for existing settings. New skip is request-time only.

Data model changes

None.