Fix MinHeap::IsValid() skipping root's children

[CodeReclaimers]

Summary

• IsValid() uses if (parent > 0) but should use if (child > 0). When child is 1 or 2, parent is 0, so parent > 0 is false and the root's children are never validated against the root.

Test program

// Test for issue 10.8: MinHeap::IsValid() skips validation of root's children.
//
// The bug: IsValid() uses `if (parent > 0)` but should use `if (child > 0)`.
// When child is 1 or 2, parent is 0, and `parent > 0` is false, so the
// parent-child relationship at the root is never checked.
//
// This test manually corrupts a heap so that a child of the root has a
// smaller value than the root, then checks whether IsValid() detects it.

#include <Mathematics/MinHeap.h>
#include <cstdlib>
#include <iostream>

using namespace gte;

int main()
{
    // Build a valid 3-element min-heap: values 1, 2, 3.
    MinHeap<int, int> heap(4);
    heap.Insert(0, 1);  // root: value 1
    heap.Insert(1, 2);  // left child: value 2
    heap.Insert(2, 3);  // right child: value 3

    if (!heap.IsValid())
    {
        std::cerr << "FAIL: valid heap reported as invalid" << std::endl;
        return EXIT_FAILURE;
    }

    MinHeap<int, int> heap2(4);
    auto* r0 = heap2.Insert(0, 10);  // root: value 10
    auto* r1 = heap2.Insert(1, 20);  // left child: value 20
    auto* r2 = heap2.Insert(2, 30);  // right child: value 30

    // Heap is valid: 10 <= 20 and 10 <= 30.
    if (!heap2.IsValid())
    {
        std::cerr << "FAIL: valid heap2 reported as invalid" << std::endl;
        return EXIT_FAILURE;
    }

    // Corrupt the heap: set root's value to 50, making it larger than
    // both children. We write directly to the record to bypass Update().
    r0->value = 50;

    // Now the heap property is violated: root(50) > left(20) and root(50) > right(30).
    // IsValid() should return false.
    if (heap2.IsValid())
    {
        std::cerr << "FAIL: IsValid() returned true for corrupted heap "
                  << "(root=50, children=20,30)" << std::endl;
        return EXIT_FAILURE;
    }

    std::cout << "PASS: IsValid() correctly detects corruption at root's children"
              << std::endl;
    return EXIT_SUCCESS;
}

Before fix: FAIL: IsValid() returned true for corrupted heap (root=50, children=20,30)
After fix: PASS: IsValid() correctly detects corruption at root's children