Revert Reusable Block Support Removal Until Later Release

Problem/Motivation

The removal of reusable block support (commit dff037f, tracked in #3516918: Remove Reusable Block Support)
was committed to the 9.1.x branch without migration tooling, creating two
problems for sites upgrading from 9.1.1 or 9.1.2 tagged releases:

1. Update hook hazard: patternkit_update_10308 throws
   UpdateException when it encounters reusable block placements whose derivative
   loader has been removed, halting drush updb for affected 9.1.1 sites.
2. No migration tooling: The removal preserved entity data but provided
   no automated path for converting reusable block placements to inline equivalents. Entity
   data is preserved but orphaned — placements render as Drupal’s
   broken block.

The deprecation change record (#3517440) states removal in
9.3.0, which is a typo — the code-level @Deprecated
annotations target 9.2.0. The change record needs correction regardless of
this issue.

Steps to reproduce

1. Install Patternkit 9.1.1 and enable reusable blocks
   (enable_reusable_blocks: true in
   patternkit.services.yml).
2. Place a reusable Patternkit block at the site level via
   admin/structure/block.
3. Upgrade to the current 9.1.x-dev (post-dff037f).
4. Run drush updb.
5. Observe: patternkit_update_10308 throws
   UpdateException because $block->getPlugin() returns
   core’s broken plugin (the derivative loader for UUID-based reusable
   blocks was removed).

Proposed resolution

Revert commit dff037f to reintroduce the deprecated reusable block code for
the 9.1.3 release. This provides a migration window for sites with existing reusable block
content before the committed removal in 9.2.0.

1. Revert dff037f — restores 24 files: derivative
loader, ReusableBlockHooks, deprecation warnings,
patternkit_requirements(), block form elements, and associated tests. The revert
applies cleanly with zero merge conflicts.

2. Guard patternkit_update_10308 — change the
instanceof check from throwing UpdateException to logging a warning
and continuing.

3. No-op patternkit_update_10309 — prevent the
premature drop of the reusable column. The column should not be removed until
the actual removal release (9.2.0).

4. Verify deprecation annotations — ensure all
@Deprecated and @trigger_deprecation annotations consistently
reference 9.2.0 as the removal version.

The reintroduced code is disabled by default
(enable_reusable_blocks: false in patternkit.services.yml). Sites
must explicitly enable it.

Change record updates:

• #3517440: Correct the removal target from 9.3.0 (typo)
  to 9.2.0. Add a note that 9.1.3 reintroduces the deprecated code as a
  migration window.
• New change record for 9.1.3: Document the deprecation-with-window
  approach: reusable blocks are deprecated and disabled by default, migration window
  provided before 9.2.0 removal.

Remaining tasks

• Revert dff037f.
• Implement patternkit_update_10308 guard.
• No-op patternkit_update_10309.
• Verify deprecation annotations reference 9.2.0.
• Update change record #3517440.
• Publish new 9.1.3 change record.
• Review and test the merge request.
• CI validation.

User interface changes

The reusable block checkbox and deprecation warnings in the block configuration form are
restored for sites that enable enable_reusable_blocks: true. No visible change
for sites using the default (disabled) configuration.

API changes

The PatternkitBlock derivative loader restores UUID-based derivative
definitions for reusable block entities. All restored code carries
@Deprecated annotations targeting removal in 9.2.0.

Data model changes

The reusable column on the patternkit_block entity is retained
(the premature drop in patternkit_update_10309 is converted to a no-op). The
column will be dropped in the 9.2.0 removal release.

Release notes snippet

Deprecated reusable block support has been reintroduced in 9.1.3 to provide a migration
window for sites with existing reusable block content. The feature is disabled by default.
Sites that previously enabled reusable blocks should use this window to migrate content to
inline block placements before the committed removal in 9.2.0, which will include
automated migration tooling.