Rename Canvas panel

Changes done locally. Pushed, pipeline running

because it allows us to just rename the plugin label and the documentation,

I didn't know "Canvas" term has muscle in so many parts of the codebase (test names, HTML class names...). I was expecting it to be only in some user facing strings.

The diff may be scary ( 115 files changed, 726 lines changes), but most of the changes happened in comments, not in proper code. And 111 of the changed lines (15% of total) are displayBuilder.canvasFrame() calls.

What have been done

In code, in the order of importance.

User-facing:

• BuilderPanel.php: the island label and iframe title "Canvas" → "Builder", and the shortcut help "Show the canvas" → "Show the builder".
• Descriptions changed in ComponentLibraryPanel.php, PreviewPanel.php, ViewportSwitcher.php and HighlightToggle.php ("…for the canvas panel" → "…for the builder panel").

Backend code:

• Class CanvasController → BuilderController, with its method canvas() → builder().
• ThemeHooks.php: the private method isCanvasPage() → isBuilderPage()
• Route display_builder.canvas_island → display_builder.builder_island, with its path /display-builder/canvas/{…} → /display-builder/builder/{…} and the controller reference.

Frontend code:

• Libraries canvas_frame → builder_frame and canvas_frame_inner → builder_frame_inner, with their file paths, plus canvas_query.js → builder_query.js in display_builder.component.yml.
• 5 asset files renamed (builder_frame.js/.css, builder_frame_inner.js/.css, builder_query.js).
• CSS classes and DOM events: db-canvas-frame (with --buffer and -wrap), -preview (with --buffer), -drop-overlay, -insert-line and -refresh all became db-builder-*. The custom events db-canvas-refresh and db-canvas-dragend became db-builder-*.
• Public API: Drupal.displayBuilder.canvasDocuments() → builderDocuments() and afterCanvasRefresh() → afterBuilderRefresh().
• Behavior and once() ID: Drupal.behaviors.displayBuilderCanvasFrame → displayBuilderBuilderFrame, and the once('db-canvas-frame', …) ID → 'db-builder-frame'.
• Expando properties: wrap.dbCanvas → wrap.dbBuilder and builder.dbCanvasMirror → builder.dbBuilderMirror.
• Sortable group name: 'canvas' → 'builder' in builder_frame.js.

Tests:

• Kernel tests: CanvasControllerTest → BuilderControllerTest (class, ->builder() calls, library names, the $canvas_page variable), and the expected URLs and libraries in BuilderPanelTest. ProfileViewBuilderTest and a few others have names in assertion messages or variables changed.
• Playwright: Spec file: canvas_frame.spec.ts → builder_frame.spec.ts, and many occurrences in the file.
• One word in each test module: display_builder.profile.test_frame.yml, display_builder.profile.test_secondary.yml and test_input.component.yml.

What have been kept

Mention of off-canvas mechanism from Core, and mentions of Acquia Canvas in FAQ.

Has been shipped in beta9