Add PhpEngine cache shim

[dereuromark]

Summary

This backports CakePHP 6.x's cache PhpEngine into the shim plugin for CakePHP 5.

It adds:

• Shim\Cache\Engine\PhpEngine
• focused test coverage
• docs for installing brick/varexporter
• a local benchmark script for comparing File vs PhpEngine

Notes

brick/varexporter remains an app-installed dependency for consumers using this engine.

Verification

• vendor/bin/phpunit tests/TestCase/Cache/Engine/PhpEngineTest.php
• php tests/benchmark/cache_engine_benchmark.php --writes=5 --reads=10
• php -d opcache.enable_cli=1 tests/benchmark/cache_engine_benchmark.php --writes=10 --reads=25

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 63.85542% with 60 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.13%. Comparing base (3594d6a) to head (ee01f7c).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Cache/Engine/PhpEngine.php	63.85%	60 Missing ⚠️
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Additional details and impacted files

@@             Coverage Diff              @@
##             master     #126      +/-   ##
============================================
- Coverage     81.40%   81.13%   -0.28%     
- Complexity      799      997     +198     
============================================
  Files            33       34       +1     
  Lines          1807     2226     +419     
============================================
+ Hits           1471     1806     +335     
- Misses          336      420      +84     

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[dereuromark]

Benchmark note:

Best-case warmed read results, 5000 reads over 100 prewritten keys:

Payload	File us/op	PhpEngine us/op
translations	16.19	7.35
model_cache	15.90	7.49
nested_config	15.79	7.22
record_list	15.49	7.22