fix: walk parent_class chain when looking up __clone for op_clone (#57)

[KorsarOfficial]

Problem

When cloning an object whose class Child does not define __clone() but ancestor Parent does, KPHP silently skips the inherited magic method — violating PHP semantics.

Formal Analysis: Method Resolution Order

Define the class hierarchy:

• parent : C → C ∪ {∅} — parent class function
• MRO(C) = [C₀, C₁, …, Cₖ] where C₀ = C, Cᵢ₊₁ = parent(Cᵢ)
• methods(C) = set of methods declared directly in C
• lookup(C, m) = Cⱼ where j = min{i : m ∈ methods(Cᵢ)}

Bug: The code used m ∈ methods(C₀) instead of lookup(C, m).

Example:
methods(Child) = ∅ → __clone ∈ ∅ = false → skip
lookup(Child, __clone) = Parent ≠ ∅ → should call Parent::__clone()

Fix

Replace klass->members.has_instance_method() (checks only C₀'s own members) with klass->find_instance_method_by_local_name() which traverses the full parent_class chain.

Correctness proof (by code inspection of class-data.cpp:268-299):
find_instance_method_by_local_name(C, m) iterates cur = C, parent(C), parent²(C), … returning the first match — exactly implementing lookup(C, m). ∎

Complexity: O(d) where d = inheritance depth (typically 1–5).

Tests

• tests/phpt/clone_keyword/105_inherited_clone_method.php — parent defines __clone(), child inherits
• tests/phpt/clone_keyword/106_deep_inheritance_clone.php — __clone() two levels up

Technical Report

📄 Full report with mathematical formalization (PDF)

Closes #57