Fix SphereBVHNode::checkCollision null deref crash on leaf nodes

[Copilot]

The const overload of SphereBVHNode::checkCollision unconditionally dereferenced left and right without an isLeaf() guard or null checks, causing a guaranteed crash when traversal reached any leaf node.

Changes

• Fix const checkCollision — rewritten to match correct traversal logic:
  • dynamic_cast guard for non-sphere colliders
  • Bounding sphere overlap test before recursion
  • isLeaf() early-out returning sphere.checkCollision(...) directly
  • Null-guarded recursive calls on left/right

bool SphereBVHNode::checkCollision(const Collider& collider, std::vector<ContactInfo>& info) const {
    const auto* otherSphere = dynamic_cast<const SphereCollider*>(&collider);
    if (!otherSphere) return false;

    if (!spheresOverlap(sphere, *otherSphere)) return false;

    if (isLeaf()) return sphere.checkCollision(*otherSphere, info);

    bool hitLeft  = left  ? left->checkCollision(collider, info)  : false;
    bool hitRight = right ? right->checkCollision(collider, info) : false;
    return hitLeft || hitRight;
}

• Removed non-const overload — not declared in the header; the fixed const version now subsumes it
• Removed duplicate isLeaf() definition and stray dead code (orphaned buildRecursive fragment) outside the closing namespace brace
• Reordered spheresOverlap to appear before checkCollision to fix declaration order

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