perf: cache ReflectionClass calls in isPhpCoreClass

[fain182]

Closes #580

Problem

isPhpCoreClass() in ClassDescriptionBuilder calls new ReflectionClass() for every dependency of every analyzed class, with no caching. This was identified in #580 as the source of a measurable performance regression introduced after v0.8.0.

On arkitect's own 104-file codebase, ReflectionClass was being instantiated 849 times — many of those for the same class names repeated across files, and many more for user-land classes that don't even exist in the current process (triggering exception handling as control flow).

Changes

• Cache: results are stored in a static array keyed by FQCN. Subsequent calls for the same class name return immediately without any reflection.
• Early exit: if the class is not loaded in the current process (class_exists / interface_exists), it cannot be a PHP internal class — return false immediately, skipping ReflectionClass entirely.
• Removed try/catch: now that we guard with class_exists first, ReflectionClass will never throw for the classes that reach it, so exception-as-control-flow is gone.
• Extracted checkIsPhpCoreClass: cache logic and detection logic are now in separate methods.

Results

Measured on arkitect's own codebase (104 files):

	ReflectionClass calls
Before	849
After	148
Reduction	-83%

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.32%. Comparing base (9092a4c) to head (e9df60e).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #607   +/-   ##
=========================================
  Coverage     98.32%   98.32%           
- Complexity      684      689    +5     
=========================================
  Files            86       86           
  Lines          1965     1968    +3     
=========================================
+ Hits           1932     1935    +3     
  Misses           33       33           

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[micheleorselli]

I left one comment on the check condition. Apart from that this seems to me a quick win to improve the current situation so 👍