MergeableNode manipulation API — tree search and edit operations

[TomProkop]

Summary

Extended API for editing the form body tree (MergeableNode). Enables programmatic form manipulation without dealing with raw XML.

Tasks

• AddControl(sectionId, control) — add control to section
• RemoveControl(controlId) — remove control from form
• MoveControl(controlId, targetSectionId, position) — move control between sections
• FindNode(predicate) — generic tree search
• FindNodesByAttribute(attrName, pattern) — search by attribute value (e.g., find all controls with datafieldname containing "vat")

Context

Today, traversing the form tree requires manual recursion:

void CollectControls(MergeableNode node, List<string> fields, string pattern)
{
    if (node.Name == "control")
    {
        var fieldName = node.GetAttribute("datafieldname");
        if (fieldName?.Contains(pattern) == true)
            fields.Add(fieldName);
    }
    foreach (var child in node.Children)
        CollectControls(child, fields, pattern);
}

This should be a first-class API on MergeableNode.

Ref: docs/roadmap.md Milestone 4.