Top bar entry for the main content display: bundle display or override

Problem/Motivation

Part of #3624760: Plan: consistent Display Builder entry points in the Navigation top bar.

On an entity page, the display built with Display Builder is reachable only as a local task in "More actions" ("Display: Full content"), with the same label whether an override exists or not. Following it on an entity without an override creates one. "Display: Teaser" is listed on the full page, where no teaser is rendered. The bundle display has no entry at all.

Proposed resolution

Add one entry to the "Display" dropdown for the display rendering full:

• the bundle display, scope "every bundle item";
• or the override, scope "this item only";
• or "Customize this item", when overridable and not yet overridden. Creating an override becomes an explicit action; the builder URL no longer creates one on GET.

Remove our override local tasks from the top bar when Navigation is enabled, keep them as tabs otherwise.

Remaining tasks

• Evaluate: list the overridden bundle display as inactive, to explain why editing it changes nothing on this item.
• Tests: bundle display, override, and overridable without override; visiting an override builder URL no longer creates an instance.