Add domain_config_entity_ui submodule — per-domain support for config-entity admin pages

Problem / Motivation

domain_config_ui supports per-domain configuration overrides through ConfigFormBase forms (system.site, etc.). It does not currently surface those overrides on EntityForm-based config-entity admin flows (block, view modes, search pages, views, …):

1. The parent module's form_alter only injects the "Enable domain configuration" toggle on ConfigFormBase and ConfigTranslationFormBase; EntityForm-based edit pages never see it.
2. Drupal core's AdminPathConfigEntityConverter loads config entities override-free on every admin route, so the edit form for a domain-overridden block renders the base values.
3. ConfigEntityListBuilder::load() calls loadMultipleOverrideFree() on the entity storage handler, so admin list pages render base values too. The same call path bites BlockListBuilder::submitForm(), where saving region or weight on a block whose label is overridden silently overwrote the existing override via the diff bridge in DomainConfigOverrideEditable::save().

Proposed resolution

A new experimental submodule domain_config_entity_ui in this project, layering five coordinated pieces on top of domain_config_ui:

• DomainAwareConfigEntityStorageTrait overrides doLoadMultiple() to fold the active domain's override on top of the override-free read path. Regular loads (saves, runtime, drush, …) flow through unchanged: the trait short-circuits when $this->overrideFree is FALSE.
• DomainAwareConfigEntityStorageInterface is an empty marker. The form_alter and ParamConverter both gate on $storage instanceof DomainAwareConfigEntityStorageInterface — capability discovery is runtime introspection rather than a hardcoded entity-type list.
• DomainAwareConfigEntityStorage is a thin shell that uses the trait, implements the interface, and extends core's ConfigEntityStorage. DomainAwareSwapRegistry::computeSwaps() auto-discovers every config entity type whose default storage_class is ConfigEntityStorage (block, view modes, search pages, views, …) and registers a swap to this class. hook_entity_type_alter() applies it.
• DomainConfigEntityUiFormHooks::formAlter() exposes the parent module's "Enable domain configuration" toggle on EntityForm-based config-entity edit pages, gated on the marker interface so types we do not cover never see the toggle.
• DomainOverrideConfigEntityConverter runs at higher priority than core's AdminPathConfigEntityConverter. Same capability gate; loads override-merged for registered configs on the active domain, otherwise defers to core's behavior.

Two-stage opt-in

1. Installing the submodule — lifecycle: experimental plus Drupal's install confirmation prompt is the gross gate.
2. Per-entity-type checkboxes on a SettingsForm at /admin/config/domain/config-entity-ui. Default install: empty. The user explicitly checks each entity type they want covered. The settings subscriber clears entity-type definitions on save so the swap takes effect on the next request without drush cr. The form opens with a yellow warning that only block has been validated end-to-end; the other auto-discovered types are best-effort and should be tested on a non-production environment first.

Extensibility — hook_domain_config_entity_ui_swaps_alter

Documented in domain_config_entity_ui.api.php. Contrib modules ship a sibling DomainAware*Storage subclass (extending the entity type's existing storage handler, using the trait, implementing the interface) and register it via the alter hook for entity types whose default storage_class is a custom subclass (image_style → ImageStyleStorage, user_role → RoleStorage, menu → MenuStorage, …). Modules can also REMOVE auto-discovered entries from $swaps to opt a vanilla-storage entity type out of coverage entirely.

Tests

Kernel (3 files, 11 tests / 49 assertions):

• DomainAwareConfigEntityStorageTest — storage swap on install, override-merged read on registered configs, BlockListBuilder save flow regression, mid-request override re-read, non-curated entity type stays on its own handler, disabling un-checks the swap.
• DomainOverrideConfigEntityConverterTest — converter capability gate (registered → override-merged, unregistered → defer to parent).
• DomainAwareSwapRegistryAlterTest — alter-registered entries surface in the registry, coexist with auto-discovery, strict-equality guard rejects mismatched contrib registrations.

Functional (DomainConfigEntityUiToggleTest):

• Toggle present on Configure block; absent on Place block; absent on uncovered user_role.
• SettingsForm flips coverage on the next request (full UX path: empty → check block → toggle appears).
• Save round-trip: toggle → register → edit label → save → per-domain override holds the new value, base config untouched, form re-renders override-merged. Skipped on environments where the parent diff bridge is missing (property_exists($baseData) probe).

Module info

• name: Domain Configuration Entity UI
• package: Domain
• lifecycle: experimental, lifecycle_link points at this issue
• core_version_requirement: ^10.3 || ^11 (uses \Drupal\Core\Form\ConfigTarget from 10.3)
• configure: domain_config_entity_ui.settings_form (Configure link on the Modules admin page)
• menu link: under Configuration → Domain → Domain config entity types
• permission: administer domain config entity ui for the SettingsForm
• dependencies: domain:domain_config_ui (no hard block dep — runtime is generic)

Related

• domain #3587744 — parent issue, write-side correctness in domain_config; merged.
• #3588057 / #3588108 — parallel "menu plugin manager cache leaks across domains" pair.