Add PK identity map to AbstractMapper, consolidate $pending

[alganet]

Replace three SplObjectStorage mutation registries ($new, $changed, $removed) with a single $pending mapping entity/operation ('insert'|'update'|'delete').

Add identity map infrastructure to AbstractMapper so any backend can leverage PK-indexed entity caching:

• registerInIdentityMap() / evictFromIdentityMap() for write operations
• findInIdentityMap() for read shortcut on scalar PK lookups
• clearIdentityMap() / identityMapCount() / trackedCount() for observability

InMemoryMapper updated to use $pending and identity map throughout. Tests added covering fetch caching, insert/delete eviction, reset behavior, and pending operation types.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 93.02326% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.58%. Comparing base (731ed33) to head (132900c).

Files with missing lines	Patch %	Lines
src/AbstractMapper.php	93.02%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #21      +/-   ##
============================================
- Coverage     98.12%   97.58%   -0.54%     
- Complexity      186      206      +20     
============================================
  Files            16       16              
  Lines           427      456      +29     
============================================
+ Hits            419      445      +26     
- Misses            8       11       +3     

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[Copilot]

Pull request overview

This PR consolidates unit-of-work mutation tracking into a single $pending operation map and introduces a PK-indexed identity map in AbstractMapper to enable cached entity reuse on scalar primary-key fetches across backends.

Changes:

• Replace $new/$changed/$removed registries with a single $pending: entity → ('insert'|'update'|'delete').
• Add identity map infrastructure to AbstractMapper (register/evict/find/clear + count helpers).
• Update InMemoryMapper to use $pending and identity map; add tests for caching and pending operation types.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/AbstractMapper.php	Introduces $pending and identity map APIs; updates persist/remove/reset behavior accordingly.
tests/InMemoryMapper.php	Implements pending-based flush and identity-map-backed fetch behavior in the in-memory test mapper.
tests/AbstractMapperTest.php	Updates reset test for pending; adds identity map and pending operation-type tests.

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