Improve TypedBinding performance with lazy handler initialization

[Copilot]

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Description of Change

Reduces compiled binding overhead by deferring property change handler evaluation until subscription time. Since compiled bindings are compiler-generated (from XAML {Binding ...} or C# .SetBinding(...)), we can optimize the internal implementation without affecting public API surface.

Key changes:

• New constructor: Accepts handlersCount and handlers function instead of pre-built array, enabling lazy evaluation
• Internal refactoring: Introduced IPropertyChangeHandler interface with two implementations:
  • LegacyPropertyChangeHandler: Maintains backward compatibility with existing constructor
  • PropertyChangeHandler: New implementation with deferred handler initialization
• Optimized re-subscription: Skips unsubscribe/resubscribe when already bound to same object
• Updated usage: Modified TemplatedItemsList to use new constructor pattern

Backward compatibility: Existing constructor remains functional via LegacyPropertyChangeHandler, allowing gradual migration.

Example (compiler-generated code pattern):

// Old pattern (eager evaluation)
var handlers = new Tuple<Func<Source, object>, string>[] { ... };
var binding = new TypedBinding<Source, string>(getter, setter, handlers);

// New pattern (lazy evaluation)
var binding = new TypedBinding<Source, string>(
    getter, 
    setter, 
    handlersCount: 2,
    handlers: source => new[] { 
        (source as INotifyPropertyChanged, "Property1"),
        (source.Child as INotifyPropertyChanged, "Property2")
    });

Issues Fixed

Performance optimization - no specific issue tracked.

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