fix(memory): remove unused WeakReference code or implement properly

[usernane]

Description

Handler::$handlerWeakRefs is declared but never populated. This is either dead code or an incomplete feature.

Current State

// In Handler.php
private static array $handlerWeakRefs = [];

// Referenced in cleanupMemory() but never populated:
self::$handlerWeakRefs = array_filter(self::$handlerWeakRefs, function($weakRef) {
    return $weakRef->get() !== null;
});

Implementation Details

Option A: Remove dead code

• Remove $handlerWeakRefs property
• Remove related cleanup code in cleanupMemory()
• Update getMemoryStats() to not report it

Option B: Implement properly

• Populate WeakReferences when handlers are registered
• Use for tracking handler lifecycle without preventing GC

Acceptance Criteria

• No dead/unused code remains
• If implemented, add tests for WeakReference behavior