task: #3618618 Preview an entity view override and a view page display on their own page

A node that renders as nothing is the one node the user cannot select, move or delete, and the builder had two unrelated ways of papering over that: a hand-written map of block plugin IDs in BuilderPanel, and a preview-only placeholder alter in PreviewPanel.

Both are gone. A placeholder is now named by whatever knows best:

• A source that can never resolve in a builder answers for itself, at any depth, through RegionPlaceholderSourceTrait. Page regions and the chrome blocks do this.
• A panel that renders a node and gets nothing back names it from the block definition, through BlockPlaceholderTrait, so Canvas and Preview agree on what is empty.

Also: a sample entity is marked in_preview, the way core's own previews do, so a formatter needing a saved ID stands down instead of taking down the render around it; and a swallowed render failure is logged rather than being indistinguishable from an empty render.

Co-Authored-By: Claude Opus 5 noreply@anthropic.com

Closes #3618618