Add Canvas Override module to Varbase Content Base recipe and install by default in Varbase Starter

Problem/Motivation

No built-in way to enable per-node Canvas layout editing, limiting flexibility for editors who need custom layouts per content item.

#3567225: Allow per-node override of Content Template via checkbox in node selector

The Canvas Override module enables per-node Canvas-based layout editing on the full content view mode for selected content types, similar to Layout Builder overrides.
#3582111: Initialize Canvas Override module

Proposed resolution

• Add canvas_override module to the Varbase Content Base recipe.

• Site admins can enable per-node Canvas layout editing per content type.
• Editors can visually compose unique layouts per node using the Canvas editor.
• Nodes fall back to the default Content Template when no override is set.

• Opt-in per content type via content type settings.
• Auto-provisions field_canvas_layout and configures displays.
• Adds a "Canvas Override" tab on nodes for editing layouts. ( which shows in the "More actions" in the Navigation Top Bar)
• Provides reset option to revert to default layout.

Remaining tasks

• ✅ File an issue about this project
• ✅ Addition/Change/Update/Fix to this project
• ✅ Testing to ensure no regression
• ✅ Automated unit/functional testing coverage
• ➖ Developer Documentation support on feature change/addition
• ➖ User Guide Documentation support on feature change/addition
• ➖ UX/UI designer responsibilities
• ➖ Accessibility and Readability
• ✅ Code review from 1 Varbase core team member
• ✅ Full testing and approval
• ✅ Credit contributors
• ✅ Review with the product owner
• ✅ Update Release Notes
• ❌ Release varbase-11.0.0-alpha1, varbase_starter-1.0.0-alpha1, varbase_content_base-1.0.0-alpha1

User interface changes

• N/A

API changes

• N/A

Data model changes

• N/A

Release notes snippet

• feat: #3582286 Add Canvas Override module to Varbase Content Base recipe and install by default in Varbase Starter