Add RefersToMorphed relation for cyclic morphed references

[roxblnfk]

🔍 What was changed

Added a new morphed relation type Relation::REFERS_TO_MORPHED together with the RefersToMorphed relation class.

RefersToMorphed is to BelongsToMorphed what RefersTo is to BelongsTo: it stores both the outer key and the target role (morph key) on the owner entity, but resolves the outer key in a deferred / "soft" way instead of as a hard "parent before child" dependency.

• src/Relation.php — new REFERS_TO_MORPHED constant.
• src/Relation/Morphed/RefersToMorphed.php — new relation extending RefersTo; adds morph-key handling (reads the target role from the morph column in initReference, keeps the morph key in sync with the related entity role, and clears it when the relation is set to null).
• src/Config/RelationConfig.php — registers the type, reusing the existing BelongsToMorphedLoader (read semantics are identical).

Note

The #[RefersToMorphed] attribute lives in the separate cycle/annotated package and is intentionally not part of this PR.

🤔 Why?

BelongsToMorphed inherits the hard ordering dependency of BelongsTo, so persisting a closed cycle through a morphed relation in a single transaction deadlocks the pool:

Pool has gone into an infinite loop.

This affects both self-references (A > A) and two-entity cycles (A > B > A). For non-morphed relations this is solved by RefersTo, but until now there was no morphed counterpart. RefersToMorphed allows such cyclic / self-linked morphed references to be saved within one transaction (INSERT + deferred UPDATE), exactly like HasOne/RefersTo cycles.

📝 Checklist

• How was this tested:
  • Tested manually
  • Unit tests added

Functional tests added for A > A and A > B > A (read, single-transaction create, detach to null) across all four drivers (SQLite, MySQL, Postgres, SQL Server). Additionally added coverage — previously missing — for the interaction of both morphed belongs-to / refers-to relations with Options::$ignoreUninitializedRelations (unset relation property is ignored when the option is true, and clears both the outer key and the morph key when false).

Full SQLite suite, unit tests, Psalm and PHP-CS-Fixer are green.

📃 Documentation

No documentation change shipped in this repo. A follow-up in cycle/annotated (the #[RefersToMorphed] attribute) and the docs site would complete the feature for end users.

[codecov]

Codecov Report

❌ Patch coverage is 89.74359% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.61%. Comparing base (8109639) to head (12afd12).

Files with missing lines	Patch %	Lines
src/Relation/Morphed/RefersToMorphed.php	88.57%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                2.x     #570      +/-   ##
============================================
- Coverage     91.63%   91.61%   -0.02%     
- Complexity     2014     2027      +13     
============================================
  Files           131      132       +1     
  Lines          5235     5274      +39     
============================================
+ Hits           4797     4832      +35     
- Misses          438      442       +4     

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