Refactor site overrides to a full-snapshot model and prepare for a 1.0.0 release

Problem/Motivation

The original implementation of site overrides composed an "effective" entity at read time by merging individual overrideable fields from a site-specific override revision onto the canonical revision (SiteAwareEntityResolver::mergeFields()). This field-by-field merge approach made several things hard:

• Sub-field-level overrides (e.g. individual values inside a compound field such as behaviors) were not possible without bespoke per-field merge logic.
• Submodules that need a "real", loadable entity for the current site (Layout Builder sections, Content Moderation state, revision history) had to work around a synthetic, never-saved "effective" entity that didn't correspond to any stored revision.
• Keeping overrides up to date when the canonical revision changes required ad-hoc logic instead of one well-defined re-sync step.

This issue also tracks the remaining work needed to publish a first stable 1.0.0 release of the module.

Proposed resolution

Move to a full-snapshot override model:

• Each site override is now a complete revision of the entity (identified by site_id), not a virtual merge. loadEffective() simply returns the latest override revision for the current site, or the canonical revision otherwise — no more read-time field merging.
• site_override_fields tracks which fields are "pinned" (intentionally diverged) on a given override revision.
• SiteAwareEntityResolverInterface::buildSnapshot() builds a new override snapshot from the canonical revision plus the pinned fields from a source revision, and invokes the new hook_sites_content_overrides_snapshot_alter() so submodules can perform sub-field-level merging (used by sites_content_overrides_behaviors for the behaviors field).
• SiteAwareEntityResolverInterface::resyncOverrides() re-snapshots all existing site overrides whenever the canonical revision is saved, so non-pinned fields stay in sync automatically (getOverriddenSiteIds() finds all sites with an override).
• Adds sites_content_overrides.install with update hooks 10001/10002 to migrate existing sites from the old merge-based model to the new snapshot model.

API changes

• SiteAwareEntityResolverInterface::loadEffective() no longer returns NULL (always returns the canonical or override revision).
• New hook: hook_sites_content_overrides_snapshot_alter(ContentEntityInterface &$snapshot, array $context).
• New methods on SiteAwareEntityResolverInterface: buildSnapshot(), getOverriddenSiteIds(), resyncOverrides().

Remaining tasks

• Core: SiteAwareEntityResolver/SitesContentOverridesService snapshot-model refactor, update hooks, schema/service changes
• Adapt sites_content_overrides_revisions_ui to the snapshot model
• Adapt sites_content_overrides_content_moderation to the snapshot model
• Adapt sites_content_overrides_layout_builder to the snapshot model; replace deprecated hook_module_implements_alter() usage with #[Hook('entity_presave')] + #[RemoveHook(...)]
• Adapt sites_content_overrides_behaviors to the snapshot model via hook_sites_content_overrides_snapshot_alter()
• Add/extend kernel test coverage for the update path and all submodules
• Update README.md with a "How It Works" section documenting the snapshot model
• Add CHANGELOG.md
• Add .gitlab-ci.yml (enable phpstan) and phpstan.neon / baseline